you mean the thing in the middle of the two zoa colonies there?
a little hard to tell, but that could be a lot of things. could be some kind of montipora, i could also buy it as a birdsnest or something similar.
i always love these kinds of weird hitchhikers. they're always fun to grow out and see what they become. some of my favorite corals came to me that way.
i'm assuming the white-ish base is hard, i.e. calcified?