When Susan’s implant gets activated and she grabs Finn and says “target acquired” it’s because that was her original mission as revealed in the Islands mini series. I hadn’t made the connection until my, like, 25-30th watch or so lol. I guess I just assumed it was because he’s the only other human around. Did everyone already know this and I’m just super slow?
Edit: forgot to mention the episode, its Preboot in S8
In the original episode featuring Susan the hoomans attack the candy people and try to eat them, cause candy is yummy. We see they are all mutants except.for Susan, whose headpiece is not removed and so we are left with no confirmation. I think it was always planned for her to be a human too.