If you can, take the whole piece of rock out of the tank to get it off and rinse it well with saltwater (that isn't going back in your tank), before you put it back.
I have one tiny little clump that comes back on the same rock but so far have been able to pinch it off w/out bursting the bubbles. A new bubble grows every month or so and I repeat the process, yours doesn't look easy to access but if you can get the stem then try that. Good luck