Cyanobacteria are single cell organisms tha are able to carry out photosynthesis. Since they lack nuclear membranes, they are properly considered to be bacteria, and not algae. In marine tanks, especially early in the development of a tank, they appear as a red slime that may grow to cover rocks, glass, substrate, and even organisms like gorgonia. They do not grow well if there is little organic matter in the water, or little phosphate, or if there is a good water current. They can be eliminated from a tank by scraping, improved water quality and good water movement. As a last resort, it is possible to add certain antibiotics that will kill them, but if water conditions are not improved they will just grow back.