I can't tell for sure what it is by the pic, but if it is cyano, its most limiting nutrients is phosphorus, iron, and molybdenum, in that order. there are many ways to get rid of it, all viable. the only one that usually works all the time is blackout. to keep it away, get rid of what it needs.
the only issue I have with blackout is that it can be pretty hard on the plants, but it works. it usually takes about 36-48 hours for the heterocysts in cyano to die off, but they are capable of nitrogen fixation, so too much nutrients and not even a blackout will kill it.