What is it attached to? Does it look like a stem?
It is definitely in the fungia family. A fralinea or something like that..
I have one that grew of a piece of rubble. I accidentally broke it off and now its growing a new one from the stem!
Lemme get a pic.. (sorry crappy cell pics)
This is the new baby forming about a week old
This is the original one I broke off accidentally(left about yr old and 1 1/2"). I just moved it for the pic but it puffs up to look similar to yours.
Top down mainly so you can see the growth stem.