Weird sack thing

[scgator]

I realize that the pics aren't the best, but....
The first is a straight on shot. It looks like a mucus sack with tiny white eggs in it. The second shot is from anangle. I thought that it might be an egg sack of some kind, but when I looked an hour later it was gone. It seemed to be anchored at least on the two edes, if not the whole bottom side.
Not worried, just curious.

 

[anonome]

Type of sponge, no worries. Just remember it is there so that if you move the rock, don't take it out of the tank. Air will kill it and polute the tank. I have a rock with zoas on it that has multiple of these sponges. They are excellent filter feeders.