So - here is a thought: Lymphocystis virus causes the fish's skin cells to become gigantic in size (unlike cancer where the cells are directed to multiply). If this fish's fins are yellow, it stands to reason that Lymphocystis enlarged cells would be yellow also. The problem with this theory is that I've seen other fish with this disease that were yellow (regal angel) and their growths were white.
In any event, the standard "treatment" is isolation until the lesions disappear. That said, this is almost never seen except in newly acquired fish. One train of thought is that longer term captives have all been exposed to it and are now immune. That would mean that you possibly could move the foxface over earlier. However, my personal quantainte process, without Lymphocystis takes a minimum of 60 days (30d copper, 14d prazi, 14 d observation)....and even THAT isn't 100%, so I wouldn't move the fish over with less time than that, and the Lymphocystis may well be gone by then.
Jay