I have a serpent star...he's kind of like having a roach in the tank to me though he only comes out when food is around and boy is he fast. I had heard that you only had to watch out for knobby starfish...and I know the red knobby ones will definately eat xenia!!
I have serpents as well and they are very good. just stay away from the ones that have a reputation for eating fish like the Green Serpent Star. I have two very big 12+ inches long red Serpents and have not seen them eat a fish. They are very cool to look at as well. The interesting thing about those Red Serpent Stars is that I purchased both of them at a reduced price because they had broken legs. The legs have long since grown back. I enjoy seeing the Serpent and Brittle stars come out during feeding time.
I have a red star that has survived the move from Atlanta which rates it as exceedingly hardy. (Only 33% of the inverts survived) It's a regular star fish not a brittlestar. It's bright red and it is somewhat knobby. It's perfectly harmless to all coral and anenomes. It's never touched any of the xenia in the tank. The only thing it's attacked and ate were featherdusters. Other than those casualties it's survived for over 2 years on extra food for the fish that made it to the bottom, ie brine shrimp etc.