I'm leaning toward a corynactis rather than the psuedocorynactis due to the size. The psuedos can get rather large (and are not reef safe as they will eat fish.) They're also fairly common hitchhikers, depending on the collection site. But, they tend to be colonial.
Hm, I might rethink it's an anemone...see if it moves or reproduces elsewhere in the tank.