Looks like the coral tissue bubbling up.
I am open minded about this, only you who has a better view can be the real judge. Could you post some better pictures close up for better review?Not a sponge....I was trying to blow it off with a baster last night and it broke......it definitely looks like tissue. Its connceted to the rest of the coral at the base of the bubble.
Plus, when I hit it with the baster, it looks like there is a film of this tissue all over the coral, and its just bubbling in those areas.