Agree with xystic, pest anemone..probably a majano. There are a couple of products to treat these guys (Aiptasia X , F Aiptasia....yep that's really the name). There are other ways to0. I would try to get rid of it ASAP as they can proliferate and be a nuisance.
Needle and syringe and inject with pure fresh squeezed lemon. With Aptasia x they seem to come back eventually for me.
I think technique is important with Aptasia X for sure. If I would let the aiptasia wrap its tentacles around the needle (they will do that if you just allow a few drops to come out as your approach the aiptasia) and then cover the sucker completely they didn't come back for me.
I did everything by the book and just as you said. Maybe I got a bad bottle but it just didnt work for me. So I wont be spending my money on that product especially since lemons are cheaper.
Thank you guys for all the tips. Btw do u guys know what kind of zoanthid that is? The name of it by chance?