Yep bubble algae -- do NOT pop it taking it out unless you want a bunch (guilty from experience).
After having zillions, I scoop them up during my water changes if I can't pick them off by hand using just the hose portion to pry them off.... then if they break all the spores get sucked up anyways (or 99.9%). I've made great progress but I've always pulled out some golfball sized ones -- seriously.