I've had this live rock from Florida in my tank several months and have this critter on the rock. Before my tuxedo urchin got to it, it had yellow petals on it and overall it appears to be in the shape of a clam, although I've not seen it open or close. Not sure if the yellow petals are algae or what, but really have no clue what it is. It's about 0.75 the size of a dime across