These are very delicate stars. Most will die in our tanks.
If you have had it less than a month, it is acclimation shock
If you have had it say 12-18 months (or less in a small tank) it is starvation.
These stars need:
Pristine reef parameters, esp specific gravity at 1.025-1.026
Mature 6 month old reef tank
A minimum of 200lbs of high surface area LR in a 150 g tank
a long acclimation time - 3 hours or so, using a drip method.
Please post your parameters, tank set up, inhabitants, etc. Seastars have the ability to regenerate but it depends on what is causing the problem. Also post a pic if you can.