I had a pair of harlequins. They only eat starfish. I would see them come out at night every once in a while but during the day they would hide in the rockwork. They need to be fed about once a month (Chocolate chip stars work fine) and if they eat it before you realize they need another starfish, they will let you know by coming out in broad daylight and roaming all over the tank looking for food.
just my 2 cents