How long did you have the star?
Most of these guys starve to death in 9-12 months, or sooner in smaller tanks. They appear to be disintegrating. This could be mistaken for being attacked by another animal, but it is unlikely - unless, as mentioned, the star was sick. In that case, many animals will start feeding on it as they can "smell" it is sick. Considering the type of star (and unless you have big nasty non reef safe hermits) I would say that it most likely was dying from other causes. Unfortunately they fate of the majority, IMO, of sand sifter stars
I don't recommend getting another.