bubble algae, a rather bothersome nuisance because few things (for sale to us hobbyists at least) eat it.
If you can, take the rock of and very carefully remove the bubbles from the rock without popping them. If they do pop I'd rinse the rock in tank water (not in your tank). If you cant take the rock out try to very carefully remove them, again without popping. They are filled with spores, popping them would be counter productive.
