it is definitely dying. How long has he had it? if its new, it was probably aclimation shock, they have to be aclimated very slowly.
If its older, it most likely starved to death, sand sifting starfish usually always die in a tank because there is not enough food for them to eat. If it is dissolving, i would give it absolutely no chance at living. So i would recommend getting it out of the tank. If it buries itself and dies under the sand then it could cause a large ammonia spike.