alot of things. It could be
bubble algae, or it could be bubbles of air that are comming in from a powerhead, filter, or skimmer. Or it could be from the substrate, gas pockets getting stuck on their way up to the surface. If it is bubble algae, (Green oblong bubbles) you can either get some
mithrax crabs, as they like to munch on it, or you can pull the rock out that has it on there and peel it off, and then rinse it off really well before you put it back in the tank. If it is oxygen bubbles, your filtration just needs some fine tuning, and if it is the exchange of gasses getting trapped you probably are not vac-ing your tank well enough.