It looks like green wire algae. I had that in my tank for several months (at about the same age as your tank) and struggled to control it. When I also started seeing green bubble algae, I put a couple of emerald crabs in my tank because they're known to eat bubble algae. But as it turned out, the crabs didn't touch the bubble algae but they loved eating the green wire algae, and after several weeks it was completely gone! I still have small patches of bubble algae that the emerald crabs won't touch, but luckily it hasn't really spread.