This star is a sand sifter, and will prefer at a maximum crushed coral. larger gravel is tricky...it should spend time burried in the substrate. Unfortunately, in many if not most cases, this star is doomed in your size tank. They eat the beneficial critters in a sand bed, and then tend to slowly starve in 9-12 months. Sometimes they may take to eating larger meaty foods, but unfortunately this does not always happen...or people don't know to take special care in trying to feed them. So you might want to give that a go.
FYI: These guys typically die by a classic disintegration...bits of arms and then whole arms start to fall off. Many people think it is some predation or something, but this is typically their normal "method" of dying.