Overgrowth of this organism "cyanobacteria" which causes the blue-green algae-like appearance generally occurs when there is a high (DOC) Level in the aquarium water; these are high levels of dissolved wastes and nutrients, also known as "Dissolved Organic Carbons" or (DOC). There are several reasons for an outbreak to occur. It may be due to lack of water changes and regular maintenance, over-feeding, lighting issues, or because the tank is new and the beneficial bacterial colonies have not become established. However, because cyanobacteria can adjust its own nitrogen levels in the aquarium water, it can appear even in a well maintained matured tank. Some cures would include physically cleaning and removing it from the tank, by doing a series of partial water changes, putting your aquarium lighting on a scheduled timer, and treating it with erythromycin which is the main ingredient in most freshwater liquid or pill form algae remedies. However when using any chemicals like erythromycin use caution, following the manufacturers directions and don't overdose. Remember that these types of chemicals can kill off your good bacteria in your established aquarium, causing further problems by upsetting your aquariums natural biological cycle. If such treatment is used, monitor ammonia and nitrite levels closely for several weeks. Best Regards, Team OCReef.com