got this from a reef central article in rk magazine
A few species of stars may be kept successfully in aquaria. Probably the most ubiquitous of these are several (?) small species of cushion stars, possibly in the genus Asterina. These small stars are gray, white, or sometimes mottled with green, and are about one half inch across. They reproduce by fission, and are seldom seen with a complete array of arms. There appear to be three distinct types, which may be different species, found in reef aquaria. The most common variety is one that appears to eat algae and surface films. The second most common variety (although it is quite rare) eats zoanthids and soft corals. The rarest variety of these small white stars eats stony corals. Fortunately, aquarium control of them is pretty easy. They are not the speediest of animals, and if you find you have a type that is causing problems, periodic starfish safaris can generally rid a tank of them