Honestly what else could it be? I know most common hitchhikers. I bought like 7 different corals about 3 weeks ago so thats where this must come from. I mean alveopora or corals in general dont reproduce right?
I've tried Aiptasia-X - on Aiptasia - and here's how I recommend using it:
Open the package of Aiptasia-X, and take out the syringe and bottle. Fill the syringe with kalkwasser paste. Cover the problem anemone with that, clean the syringe, and throw out the bottle of Aiptasia-X.
(The stuff not only didn't work for me, it allowed Aiptasia treated with it to grow to massive sizes.)