I have NO idea, but you may want to try posting the pic on reefcentral.com - there are loads of people there, maybe one of them has seen this strange critter before. If you can add a description it might help as well - does it move? Change shape at all? Is it hard, soft, smooth, rough? How large is it? Anything you can tell us about it might give someone a clue. Good picture, I hope someone can ID it for you.