In some sense, the analogy between morality and arithmetics is right. On the other hand, the meaning of arithmetics can be described enough precisely, so that everybody means the same thing by using that word. Here, I don't know exactly what you mean by morality. Yes, saving babies, not comitting murder and all that stuff, but when it comes to details, I am pretty sure that you will often find yourself disagreeing with others about what is moral. Of course, in your language, any such disagreement means that somebody is wrong about the fact. What I am uncomfortable with is the lack of unambiguous definition.

So, there is a computation named "morality", but nobody knows what it exactly is, and nobody gives methods how to discover new details of the yet incomplete definition. Fair, but I don't see any compelling argument why to attach words to only partly defined objects, or why to care too much about them. Seems to me that this approach pictures morality as an ineffable stuff, although of different kind than the standard bad philosophy does.