I had a (smaller) aiptasia on growing in between my Zoa's that I left there for almost two weeks (one week to notice, one week to decide a course of action). I took my zoa frag out, pulled the aiptasia off, rinsed the zoa before I put it back, and 3 months later none have shown up anywhere else.
Not sure if that is the best course of action to follow (I'm sure someone else can give you better advice) but at least in my case the aiptasia didn't spread.