Unfortunately, you will eventually need to target feed choc. starfish. First, they will eat whatever they can find in the tank. If not fed enough, they will stalk and eat any fish they can get ahold of. They are omnivores and opportunity feeders. If not target feeding, I would suggest making sure enough food gets down to the bottom for them.
Many people will say most - if not all - starfishes are difficult to keep.
Brittle stars are easier, but the wrong one can wreak havoc.