Its melting, my best advice is remove it now cause they get really rancid when they melt and die, worse smelling thing in the world and I've nuked a 10 gallon from one melting in there during a harlequin shrimp feeding. It'll also leave behind these white pebbles you may or may not like visually.
Not sure on the cause in this case, could be starving, didn't take a parameter swing well, or fed the wrong foods.
The ones I keep in my sump to cut legs from for harlequin shrimp all seem to be doing well for a few months now, I give them a clam and occasionally leftovers of uneaten food from the other animals. If it got injured on the oral disk it could've gotten infected and spread to the rest of the star. Looks like from the first image that could be the case. Sadly nothing you can do at this point.
