damsels are mean, but i have never heard of them eating a star! could the star be disintegrating? they do this if the tank is not exactly to their liking. the damsel may just be being an opportunistic feeder. usually the stars are to big for a damsel to even think about making it food.
how many places in the tank can the star hide? if the only place to hide is in the damsels territory then it could just be trying to chase the star out.