Offer meaty food like squid, prawn, and/or silversides about once or twice a week. The bigger the starfish, the bigger or more it needs. The rest of the time it can spend collecting food from within the system from what is fed to the fish and on natural growth.
Reef compatibility is going to depend on the type of serpant star and how big. The green serpants do get large and I've lost small fish to them. They also have a habit of toppling things over as they cruise around. Good to well anchor coral. Can you get us a picture?