[JustDavidP]

It is Bubble Algae.. "Valonia SP". Most often, you see it in small, round, berry sized growth formations. However, it can grow to look like bananas or even linked like tiny sausage farms Emerald "Mythrax" crabs have been reported to eat it. Mine didn't much care for it. Manual removal was my only option. Careful not to "pop" them or it will spread in your tank. IF you can remove the rock, do that, and rinse with salt water before putting it back into the display.

Dave