The aliens could say it's morally right.

That doesn't seem relevant. I was noting cases of what the aliens should say based on what they apparently wanted to communicate. I was thus assuming they were speaking truthfully in each case.

In other words, in a world where strong moral relativism was true, it would be true that the aliens were doing something morally right by exterminating humans according to "their morality". In a world where moral realism is true, it would be false that the aliens were doing something morally right by exterminating humans, though it might still be the case that they're doing something 'shmorally' right, where morality is something we care about and 'shmorality' is something they care about.