if he has white spots on him, and pieces are missing, from what i've ready on WetWeb, it's likely dying. do the edges around the missing parts look cut, or like it's falling apart?
also, another way to check touch it and see if it's soft... that's another indicator it's going downhill. sea stars should be firm, almost hard, when healthy.
if it has white spots, is missing pieces out of it, and is soft, i'd remove him from your tank, before it dies and pollutes the water.
just because he was at the store, and ok, doens't mean he made the transition to your tank ok...
did you check the water parameters in the acclimation container and your tank to make sure they matched? also, do you have big temperature swings in your tank daily? sea stars are actually more sensitive to temp swings than anything else. 2 hours is a bit on the short side for these guys, i acclimated my blue linckia over like 4-5 hours.