It is blue green algae. Keep your tank clean. If you live by wet lands you will get the spores in the air. The way to totally wipe it out is to suck out all you can then use erythromycin, Maracyn by Mardel, at 1/2 strength for disease, one time. This does not harm the fish, plants or biologicals and does not cause resistance. Blue green algae is a bacteria that drifts in the air or is passed directly from a supplier. It does not get out of control unless you don't see it and/or your tank is dirty. Be sure to bleach everything that touches that tanks water (1tsp bleach per gallon of water) including your arms and hands as you will pass it on to another tank or bring it back to the tank to the same tank a few days later.