Looks to be a torch or hammer coral spawning. Do you have one of those in the tank?
Edit, might be a majano, disregard my previous message. Looked them up and didnt realize they ended up looking like that, Ive only seen them really small.
Yea I'd agree with the stony coral ID. Some fungiids like Heliofungia and Cycloseris do have decently plump tentacles and the "juveniles" look an awful lot like the polyp you have.
One thing for sure, it's not a majano since the tentacles aren't arranged in defined growth rings. I'd say if it's not immediately bothering anything, let it grow out. Who knows you might just have a crazy cool hitchhiker