From your pic, it looks like your starfish is suffering the same fate as virtually all home aquarium starfish...they simply dissolve in saltwater tanks. I read about this phenomena a year or two back in Coral magazine. The jist is that starfish are outrageously sensitive to very slight changes in salinity; the starfish surface you see is very thin, and the starfish itself is basically a sack of carefully controlled osmotic salts that quickly become depleted when the osmostic pressure on the cell walls changes. Salt either rushes out of the starfish when it's unbalanced, or rushes in when its unbalanced, both causing death and dissolving of the starfish. Early on, to save itself, it jettisons an arm in an attempt to stop the loss of its internal fluids.
Linkias really shouldn't be sold in the aquarium trade, along with most all starfish. Despite ato's, humans cannot replicate in a couple hundred gallons of water the exact conditions of the ocean which are amazingly stable to allow starfish to survive. Over time, an inmbalance will inevitably occur and the starfish will die. Of course there are plenty of anecdotes about starfish living years in larger established aquariums; but for every 1 that survives, 1000 dissolve. Good luck though.
