"Well-being" is a know-it-when-we-see-it sort of thing. Sure it's vague, but I don't begrudge its use.

Let's break down the phrase you just objected to (I have not read SH's book, if that matters): "Increasing the well-being" - roughly correlates with increase utility, diminishing suffering, increasing freedom, increasing mindfulness, etc. Good things! And if defining it further gets into hairsplitting over competing utilitarianisms, then you might as well avoid that route. "Of all conscious creatures" - well, you obviously can't do anything immoral to a rock. Maybe you kick a rock and upset the nest of another creature, but you haven't hurt the rock. But you can do immoral things to conscious creatures, which can be argued to be pretty broad; certainly broader than just humans.

So I think this is as concrete as many one-sentence summaries of morality.