If it moves in the current, closes up like the zoas, and doesnt have a stony base, then its very very unlikely that its a stony coral. Probably some kind of anemone, though which one will be close to impossible to tell Im afraid.
Ive noticed a lot more interesting hitchiker anemones coming in with zoas lately. We actually had a couple juvenile Hell Fire anemones come in a couple months back at work, and theyve been slowly multiplying in the tanks. The odd thing is- like yours- it doesnt appear to have much of an effect on the surrounding zoas. Ive noticed that with a few different hitchiker species.
Im gonna have to set up a little species tank for one one of these days.