Daugherty is right, snails and hermits won't attack and kill a healthy starfish. If it's already dying and has flesh hanging off, then yes, that's their job to clean that sort of thing up.
Like others have said, starfish are usually good at regenerating lost body parts. However, I've had a starfish die, and it just slowly rotted away until it had no legs and was just a bouncing stump of a body (it was still alive). Quarantining it is your best bet, but there isn't anything you can do to help it recover. I also agree with Daugherty that the anemone may not have been the reason for it getting sick, it's hard to tell. Maybe, maybe not. Good luck.
