I've dealt with this on my pair of clownfish - the only thing that works for it is to make the environment less stressful for the fish, and overall just make it happier.
If your QT tank is lacking in places to hide, add rock or plastic pipes. If water quality is an issue, then work to improve it. If the fish spooks easily, cover the tank a bit to minimize view outside the tank.
Basically, it's about improving the overall health of the fish. That's just about the only way to treat lymphocystis. It takes a while to clear up too. And watch out for secondary infections caused by a weakened fish.
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