I'm not exactly sure what this is. I've been searching the internet for some ideas but still don't have the slightest clue. So I joined this site in hopes for solving this mystery. It's just a green "bubblish" growth in 2 patches. We tried scrubbing it with a tooth brush to see if that would do anything and it didn't budge like bubble algae would. Also it looks nothing like it. We've identified everything else (we've seen so far) except for this little number.