So I started using this mod AFTER i created the world and I dont find any alien remaains, do they still spawn or do I need an entire new world? And would it be possible to add it for already existing worlds? Like they did with the uranium (it spawned in a old world as well)

Yeah, you should be able to get it on an old world. - You just have to explore far enough. It just has a low spawn rate. :)

So is alien remains an ore or something like oil?
Cause I havent seen it yet, and I started a new world with this mod already enabled.

It's just like oil. Except it has a slightly less spawnchance than oil. Or atleast should have.