Starfish Amputee

[MMAX]

I dropped a 3 to 4" sand sifting star into my tank yesterday to see if it would take care of the detritus in my sand. Lsat night I noticed that it seemed to be pinned against a rock in the back. Checked again and it was loose but missing one leg. I'm almost positive I don't have a mantis in the tank, the cleaner shrimp, small clowns and snails would have been long gone and I've had them close to a year. I know there's a dime sized hairy crab that lives in the rock where the star was pinned but he always seemed peaceful and I find it hard to believe a crab that small would eat a 2" leg. Any thoughts? If the star pulls through how long would it take to regenerate the leg?