That's because it's a hard coral, Echinophora mammiformis. Now we need to let it grow to see if its going to be a branching or plating or encrusting kind.
how much space do u need behind ur tank. im putting mine in the wall and i need to know how far off the wall do i need to go. it will be accessed from the front. do i need room in the back or can i put it against the wall?