I don't know if they're aliens. They're hostile, but it seems almost incidental most of the time, honestly... Titans don't seem to need to eat humans. Once they've eaten their fill of humans, they'll vomit up what they've consumed and go to eat more. They don't eat other animals at all. I believe they photosynthesize, actually, as they are inactive at night.

[he didn't ASK but studying Titans is her life's work so she starts to talk about it even when slightly prompted honestly]