True to their name, sand sifting stars eat stuff from the sand. In the wild their diet consists of the critters that live in the sand, so called "infauna".
In your tank, they eat the same critters and when they run out, they starve to death.
Being that your tank is relatively new, it has not had the chance to develop any real population of critters to sustain your star. Actually, a very large tank would be required to do that..
Often, it takes much longer for the star to show the symptoms of starvation so that may not be the only contributing cause to your animals problem.
It is quite possible that it is too late for your animal but you can always purchase a detritivore kit for your sandbed. It will give it some temporary food.
It's best not to purchase these stars as they normally meet the same end.