Starfish can expel their digestive tract. I would guess that is what that is. It is usually not a good sign. Make sure you water quality is as good as you can make it. If you do water changes try to match the parameters in the tank so you don't put additional stress on it. Did you acclimate it when you put it in the tank? Starfish are extremely sensitive to changes in water parameters. Also, some cannot be exposed to air or it will kill them. Can't remember which species it is though.