I would guess that you might not have acclimatized it enough to your tank.
starfish are one of the most delicate inverts for any change in water condition. Linkias I find are the worst.( I kill an entire shipment of over 20 once because I was in a hurry and didn't take the time to properly acclimatize them)
like the others said, you will definitely be able to smell it if it is dead for a while....
I now spend at least 3 - 4 hours acclimatizing them with a air hose in a slow drip into their container. I then float that container after that so that it is the same salinity and same temperature.

HTH
Neal

unfortunately most starfish slowly starve to death over the course of a year or two.

i think I disagree with that.
I know that whenever I put the dried algae, the stafish are all over it.