What are your water parameters (actual levels, including specific gravity, pH, alk, ammonia, nitrite, nitrate)? There are a lot of things in such a young tank. How did you cycle the tank? How much LR do you have? How long have you had the star and how did you acclimate it?
All in all, this sounds very much like acclimation shock, or osmotic shock, which occurs within a month of introduction, or after a large water change. There is little that can be done to reverse it.
IMO, your tank is much too young for a seastar.