DON'T POP IT.

I cannot believe nobody has said it yet. Basically bubble algae fills with spores causing it to expand. When you pop it you release thousands of spores into your tank. You must very carefully remove it from the rock without popping it.

You know the answers are the standard lights out and heavy/frequent water changes.

Good luck.

Any chance you feel it's from using natural light? I'm adding the mechanical butterfly dampers to mine so I can control the amount of light that comes in every day.