This stuff is nasty, but one thing I can tell you is that if you keep trimming it or popping those they just spread even more. You must remove these without popping them. In my past 10 gal I had like 3 of them, I carefully took out that piece of rock, over to the sink I went and with a knife I carefully got underneith them and they came off fully intact like bubbles and did not burst. I never had anymore from that time on and it was 4 years from that time.
Hate to tell you this but Tangs will not touch this stuff, they look for soft algae they can pick off the rocks. He is cute though, enjoy him and if you don't have alot of the algae he would normally eat do make sure you supplement him with some seaweed sheets, they love these and are very good for them. You can pick them up cheaper at the health food stores than the pets stores, aslo called Nori . They come in red, green or brown sheets and Tangs will just love you for this along with regular fish foods.
Hope this helps and you can get this under control, enjoy your new fishy too I love tangs.....
Amy