hmmm... not sure. Usually I try to avoid egglike things that send out spores (explode), because they're not usually good. If nobody else chimes in with an ID on this thread, you might want to post up a new one with the best pics you can get of it. Just say you need an ID on it, that it looks like an egg that's exploding or sending out spores. There's a lot of good folks in the club, most of whom are better at identifying those kinds of critters than I am. If it really bothers you, take that rock out of your tank and see if you can remove the egg, then rinse it well to get all the egg bits off the rock and put it back in your tank. It might be harmless, tho.