Welcome to ReefSanctuary!!
We need more information. Please post the following water parameters:
- Temp
- NitrAtes
- PH
- Salinity
- Ammonia
- NitrIte
Maybe the arrowcrab is chomping on it? Arrow crabs are notorious for eating anything they can catch!!

How long have you had the starfish?
What type of water circulation/filtration do you have?