Bubble algae - Valonia spp. - annoys people as it spreads into their aquariums. Normally deep green in coloration, these hardened bubbles spread throughout the system. Feeding off nutrients as well as lighting, if left unchecked it can choke out corals. Manual removal is one method, using forceps to pluck them off rocks or plastic fittings. Emerald crabs will consume them. If the aquarium is large enough to house bigger fish, Naso tangs and Foxface fish will eat valonia. Reduced phosphate and nitrate level will weaken this plant's grip on rock.
Valonia, also nicknamed Sailor's Eyeballs, can grow quite large. 1" or greater in diameter in fact. But usually it's about the size of small peas. It may even grow in a geometric pattern, resembling a flower with petals, but more often it is just a mass of collective bubbles.
It may also be found reddish in color, and red valonia is more challenging to erradicate. If you discover it on frag plugs, you can dip the plug itself in 3% peroxide solution, carefully avoiding getting peroxide on the coral itself. After dipping it in solution for 15 seconds, rinse it off in tank water and then place it back in the system. Within a few days, any algae on the plug will turn clear and fall away.