How old is your tank and what are your parameters?
Starfish in general are delicate and require stable parameters and a well established tank to survive long term. They don't do well with parameter swings and will dissolve (starve to death) if there isn't a well established biofilm and algae for them to eat.
They are also quite sensitive to salinity swings and if not acclimated slowly, will not do well.
Please provide us some more information...