not a good sign...could be from dirty water, or a bad acclimation, or rough shipping, starvation, etc. I am guessing it is a type or fromia star. These are incredibly sensitive animals. They require CAREFUL handling from ocean, from divers to wholesalers to retailers to home. Your starfish probably made 4-5 stops and acclimations between ocean and home. Each stop, the person needs to handle the animal carefully and drip acclimate for 3+ hours (not gonna happen from divers/wholesalers). They are incredibly sensitive to pH and salinity changes, and don't particularly like nitrates and phosphates.
They are very specific eaters and probably havent had proper food sources for days to weeks. They require LARGE tanks that are WELL established with a diverse flora to munch on. They can be kept successfully in captivity but it is rare. I recommend not purchasing these animals because demand fuels supply. I would avoid linkias also
For more info google: starfish melting or starfish falling apart, unfortunately it is quite common