Thetans are us, soul, spirit. I don’t know what an alien soul is. A soul that is from another world? A soul that is inhabiting an alien?
I don’t know anyone who has had their body taken over by an alien soul. No one has ever told me that they had gotten the alien soul out, or mentioned that they were working on getting an alien soul out. I don’t know of any scriptures that refer to alien souls in our body. This all sounds like another invention of some anti-Scientology jerks who make up stories to go “ho-ho-ho” about Scientology.