Large bubble on my hammer???

A large bubble just popped up on my hammer coral tonight, is this something to be concerned about or is it just a normal occurrence? I've had this piece for about a year now and I've never seen anything like this. Also I don't know if this helps or not but it almost seems as if you can see either the top portion of the skeleton or a mouth inside the bubble at the base.

PS: the dot in the center of the bubble is just a flat worm.

 

my frogspawn did this when its heads were splitting. Could be the same thing.

 

I saw that when my frogspawn split..Didnt last long though..

 

That could be it, its possibe I may have missed it in the past. This hammer has grown like it has been dosed with miracle grow, it was just two heads when I got it.