that looks like bubble alage take it off the rock but do not break it open it is best if you take the rock out of the tank to do that. try using tweezers.
Definitely bubble algae. I've had black and silver ones in my tank too.
I'd get rid of it before it starts to spread even more. Don't pop it in your tank, or it will release spores. They're pretty easy to remove by hand with a pair of tweezers.