Sand sifting star help

Hey TRT. My sand sifting star has been in my tank for 2-3 months now and I've recently noticed it losing legs. Half of one 3 nights ago, the 2nd half 2 nights ago, and a 2nd last night. My question is what should I do with him? I'd give him to the lfs but I'm sure they won't want a 3 legged sand sifter and I don't want to keep him in the tank if he's dying/being attacked.

Thx for any input