Moopsy from "Star Trek: Lower Decks" would be a formidable opponent regardless of whatever science-fiction creature it was up against. A xenomorph from "Alien" may have corrosive blood, but it's not going to do much good without any bones. And even though a xenomorph's biology doesn't make a ton of sense, they do appear to have bones if "Predator 2" is any indication, as the skull of a xenomorph can be found on the aliens' ship.
Perhaps it would be more fair to put Moopsy up against a similar entity from the same franchise, namely the tribbles from "Star Trek: The Original Series." Tribbles can be thought of as an early variant of Moopsy from the 1960s. They aren't quite as unsettling since they don't drink bones, but they're plenty dangerous without that skill. Instead, the tribbles aim to eat and reproduce ad infinitum. When it comes to cuteness, Moopsy probably has the advantage because it has an expressive face.
Moopsy could also probably wipe the floor with the tribbles in a battle of brawn. Tribbles can multiply and take over a spaceship quickly if one's not careful. But again, and this can't be repeated enough, Moopsy drinks bones. As rodents, tribbles are likely filled with bones, so assuming Moopsy is particularly hungry, it could get through dozens of tribbles faster than they can reproduce. While we'd like to say we hope we see Moopsy again because it's so darn cute, it very well could haunt our nightmares with the way it slurps up bones like pho.
