We never really see it eating. All it did was smash up the dead body with its inner jaw, then used the inner jaw to bring back small parts. Its never really clearly shown eating, chewing, swallowing, ect. If you don't like the fact that it was eating, you could just say it was ripping apart a dead body for shits and giggles.
Personally, I don't mind. Were just seeing stuff that was already assumed off screen. And I find it funny that people can cry about the fact that the Predalien has dreadlocks, and how it doesn't make sense, yet when it comes to an alien eating, because it needs food to live, that's totally unreal.
Aliens don't need food. I mean, it only provides them with energy and sustinence. They don't need that.