What are your water parameters, especially pH, alk and specific gravity? Did you do any recent water changes?
IMO, you tank is far too young for any seastars, and this may have been the ultimate cause of its stress and demise. The signs - the 'disintegration' - are not characteristic of an injury, they are characteristic of stress- either osmotic, or starvation. Starvation typically occurs after 9-12 months, but since your tank was so new, who knows.
I do not recommend, as a general rule, sand sifting stars which eat the good critters in live sand, and are far from beneficial, IMO.
I am not above begging
PLEASE do not get another seastar until the tank is at least 6 months old. Especially not a Linckia or Fromia, which are far more delicate than sand sifters, and have incredibly specialized diets. They require very mature LR, and lots of it. Blue Linckia, and any species of Fromia, may not be suitable choices for your tank. Most Linckia and Fromia will die of starvation 9-12 months after introduction into a tank...I really can not emphasize enough the need for selecting an appropriate species and individual for your situation.