How old is your display tank? Linkia need a mature tank with a pretty good algae film to feed upon. If your tank is fairly new, that's probably why it's not doing well.
Chances are your QT tank is an even worse environment for it, as QT tanks usually tend to be "cleaner" than the DT. If that's your situation, I'd move it back to give it the best chance. But yeah, if it's that far gone already, the chance of it surviving is very small.
Also, try not to expose it to air when moving it. Keep it submerged at all times if possible.
I've never seen a linkia being supplementally fed. In fact, it's hard to see what they eat at all. Mine cruises the waterline of my tank 99% of the time, and there's very little visible there for him to eat. But he is surviving on something.