Its dying. Probably starved or didn't do well during acclimation. How long have you had it?
Linkya starfish are too sensitive to conditions of most fishtanks, swing in temperature or chemistry and it self-destructs.
I've decided not to try keeping these and would not advise anyone.
It is an old post, so by now it is dead, right ? If that was earlier I would advise to remove it from the tank, so not to faul-up the water.
In the whild it would recover (these even can grow into new from cut-off leg, unlike other starfish) , but you can't provide it with such conditions. Sorry.