Green bubble algae. It will get worse if you don't remove it. I tried plucking it but you have to do it without breaking the bubble.
I got some emerald crabs, which are amazing grazers, and they ate it all, and they love macro algae, which I also had. But I really wanted the emeralds, you could take the rock out and pluck it off in old saltwater, then rinse with more saltwater and put it back to see if you got it all off.