"Exterminating us is wrong!" ... and the alien could say: "LOL. No, silly humans. Exterminating you is right!" And there is no sense in which either party has an objective "rightness" that the other lacks. They are each referring to the utility functions they care about.

Note that the definitional dispute rears its head in the case where the humans say, "Exterminating us is morally wrong!" in which case strong moral relativists insist the aliens should respond, "No, exterminating you is morally right!", while moral realists insist the aliens should respond "We don't care that it's morally wrong - it's shmorally right!"

There is also a breed of moral realist who insists that the aliens would have somehow also evolved to care about morality, as the Kantians who believe morality follows necessarily from basic reason. I think the burden of proof still falls on them for that, but unfortunately there aren't many smart aliens to test.