Green bubble algae

[Enigma]

I had to remove one bubble.

I pulled the (so small it fit in a coffee cup) rock, plucked off the bubble (rupturing it in the process) and then set the rock in a mixture of 1:10 hydrogen peroxide to saltwater for 10 minutes to kill any spores.

That was four weeks, or so, ago. I haven't seen any others since then, but I think the time span is too short to know if it truly worked or not.