Cyanobacteria can be caused by excess nutrients due to overfeeding, using tap water etc...anything that contributes to high nitrates and/or phosphates. While not really harmful to the tank, it is ugly and if left unchecked can choke out macroalgae growth and get onto corals and other sessile inverts causing irritation. The best treatment is to find out what is causing the excessive nutrients and get that under control.
Do you test your water for nitrates and phosphates? Post some test results. What do you use for make up/topoff water? RO
is best for keeping nuisance algae in check. Do you run a skimmer? How long has the tank been up and running? Post some tank info and you may get a better idea of what is going on.