Per author's request, I am not discussing the allegory. My discussion of the hypothetical as presented does not imply endorsement of any kind of allegory. A corollary of this is that by proposing a solution I do not endorse any arguably analogous solution in the real world. Also I'm ignoring the whole mothership thing because that hasn't been elaborated enough to discuss without importing lots of assumptions based on the analogy.
With that said...
One big question is what the net human utility from martian tickling is. If it's net positive, we should definitely encourage more tickling. If not, things get hairier; we could look at total utility, but this rationale makes Martians a bit utility-monstrous, so it might be preferable to find or create an acceptable way for Martians to compensate humans for tickling privilege.
Our key concerns will be probably be utility and fairness, both for humans and for Martians. It's pretty important to make sure Green Martians get a chance to practice. One option would be for all humans to agree to make themselves available for a certain number of Green tickling; a solution requiring less global coordination would be for humans to be able to pair with Green Martians under a binding contract that guarantees a certain amount of post-metamorphosis tickling in exchange for allowing the Green Martian to practice enough to metamorphose. This latter arrangement has advantages of also allowing slow-learning Green Martians to negotiate compensation deals with their human partners if that's allowed, and the human and the Martian can decide what tickling techniques will be net wins.
But these are global changes. If the world doesn't make sense, what's a compassionate Green Martian to do? One option is to try to find humans willing to consider alternate tickling norms and reach agreements with them. If that doesn't work, then if tickling is generally a net increase to human utility, I'd say it's probably fine to go out and tickle the way Martians tend to, since humans implicitly expect this. Adopt the techniques iff you think doing so will be a net utility win. The ethics of spreading info on the techniques depends on your best estimate of the difficult empirical question of whether they'll be used well or poorly.
EDIT: Elsewhere in the comments the hypothetical is modified to specify that explicitly-consensual tickling is ineffective. The best solution is then to leverage implicit consent as much as possible. If being in a particular place at a particular time, or wearing a marker, is sufficiently implicit, then the all-humans-make-themselves-available-for-tickling solution can work. The pairing solution can only work if there's a way to pair without explicit tickling-consent: for instance, if blanket permission to tickle in the future doesn't count as explicit consent for any particular tickling instance.
Also, some key issues not specified in the hypothetical: Do martians and humans differ in their (dis)utilities from tickling, and does each martian/human's (dis)utility vary with time and mood? If so, we'd want to incentivise higher-utility ticklings by e.g. letting humans choose times not to be tickled or giving especially Green-averse humans a way to opt out of Green tickling and compensate the rest of humanity somehow. Do martians care who they tickle, and do humans care what martian tickles them beyond the color? If so, there's a match component that we want some way to optimize for. Does marginal (dis)utility of tickling change, for either species, based on how many ticklings they're involved in? If so, this is something to be strategic about; we could either choose a few volunteer humans to be green-tickled for a living, or try to ensure that green-tickling is as evenly spread as possible. Do martians have a desire to metamorphose independent of whether this will allow them to tickle more? If not, we need to ensure there's some incentive in place for them to do so. I have intentionally refused to import answers to these questions from across the analogy.
Green Martians and Blue Martians have one thing in common: They both derive a tremendous amount of utility from tickling humans behind the ears, using their soft, feathery tentacles. In fact, the utility that they derive from this is so intense that most scientists believe at some time in the recent evolutionary past, there must have been a large selection pressure directed at ensuring that Martians were motivated to tickle humans.
There are numerous differences between Green and Blue Martians. One of those differences is that whereas the feathery tentacles of Green Martians contain stinging hairs similar to nettles, the analogous anatomic part of the Blue Martian contains a safe drug with an euphoric effect. Therefore, humans who are tickled by green martians experience a moderate stinging pain, whereas those who are tickled by blue martians experience mild to moderate pleasure.
Human ethicists have long struggled to come up with a coherent ethical theory that determines whether tickling humans is morally acceptable. Some have suggested that tickling humans behind the ear is ethically permissible if and only if you are a blue martian. However, many other thinkers are worried that this line of thinking results in an unjust world, where the ethics of an act is determined by characteristics of the Martian that they cannot be held responsible for.
However, human ethicists are not very familiar with Martian physiology, and the situation is actually even more complicated than they suspect. In fact, all Martians are born Green. They can shed their green shell and become blue Martians only after they have perfected the art of tickling humans with their feathery tentacles. All Martians aspire to one day become blue, but the amount of practicing it takes to reach perfection is highly variable - some martians reach perfection at their first attempt, whereas others keep trying their whole life without making any discernible progress. Therefore, if the ethical code says that green martians are prohibited from tickling humans, ethical Martians will be unable to reach their full potential in life, and will be stuck as Green Martians forever. Under this ethical code, only unethical Martians will be able to metamorphose.
Making the situation even more complicated, is the fact that a group of recently metamorphosed Blue Martians are vocally spreading information on the internet about tickling techniques. These techniques are sometimes effective, but if used imperfectly they increase the sting of the stinging hairs fourfold. Importantly, it seems that part of the reason some young Green Martians are naturally better ticklers and therefore metamorphose earlier, is that they intuitively understand these techniques, and are able to apply them without increasing the sting of their tentacles. Moreover, while the tickling technique has empirical support, the theory behind it relies heavily on speculation about human evolutionary history that may not be true, and which is offensive to humans.
This raises a number of additional ethical questions: Is it unethical for a Green Martian to attempt to metamorphose? Does this depend on whether they believe themselves to be fast or slow learners? Should only the small subset of Martians who intuitively understand the tickling techniques be allowed to use them? Is spreading explicit information about the techniques unethical?
(Note : This parable is obviously an allegory for something. Discussing whether the allegory is valid is interesting, but will lead to mindkill. I would prefer if the discussion could stay focused on the Martians, so that we can discuss the ethics of a hypothetical scenario that may not be relevant in real life. I am genuinely confused about the ethics of this, and I think this can lead to an interesting question regardless of whether it is applicable to humans)