There are basically two types of green microalgae, one that coats the surfaces of aquarium side panels and substrates in the aquarium, and one that 'infects' the skeletons of stony corals (Ostreobium spp.). Both are brought about by excesses of nutrients, e.g., phosphate and various nitrogen containing substances such as nitrite, nitrate and ammonium. The more common sheet-like green microalgae can become problematic if allowed to cover wide areas, but can be controlled by improving water quality and adding some animal species that will feed upon it. In some cases, it's a good food for some herbivores animals. However, once stony coral skeletons become infected, the skeleton material becomes soft, the coral waste away, and there is no known way to halt the spread of that alga.