Different types of star fish eat different things. I have had these Formia types and they need a designated area where film algae/blue green algae/bio film is allowed to grow and flourish. For me I refrained from clean certain panels of glass to allow the star fish to graze in those areas.
I am guessing your tank is too clean and too new to sustain a star fish. It is certainly possible that the star has been slowly starving to death, and was probably doing so before you purchased it.
Point is that not all star fish eat sponges, some are active predators that eat corals such as the chocolate chip, or algae eaters. Sand stirring stars eat the flora and fauna of sand beds including detritus. Some do in fact eat sponge but I doubt that star is an exclusive sponge eater.