Unfortunately Fromia starfish (as seen in my avatar) are one of those creatures that's stunning, but not suitable for long-term captivity. They generally starve to death in all but the largest aquariums (300 bare minimum...500g may have a decent chance) within a year. There isn't a whole lot of knowledge as to what these starfish eat...or why they aren't eating the food source if it's already in our aquariums. Starvation will show as leg-tissue necrosis...and eventually all the legs will fall off, and the starfish will die. Sorry to paint such a gloomy picture.
As far as it not moving much, it's hard to say. It may still be acclimating...or not. Mine moved around a ton, even when it was dying. Other than checking all of your parameters, I don't think there's anything you can do for it. Good luck.