Unfortunately, moving to a QT without acclimation is a very bad idea. What are your SPECIFIC TANK PARAMETERS in the QT (and the display tank). As mentioned, I also believe the star is dying from acclimation shock. In addition, your tank is pretty young for a star of any type.
I do not give up quickly on these stars. I have seen some in dismal shape (all arms lost) and now they are very very healthy. But in order to help, we must get your parameters into the ideal range. At the very least, this means a specific gravity of 1.025-1.026. If you are not at this level, it is advisable to slowly raise this by topping off with saltwater instead of fresh.