sounds like bubble algae to me
be careful not to pop th ebubble or else you can release thousands of littel spores into the water causing a horrible bloom. id reccommend getting an emerald crab, or if its localized right not po the bubble underwater undera siphon tube and vacume all the junk and excess tissue out of the water.. or just leaveit alone. a couple of them in a tank can be kinda attractive!
good luck
jon