Fallow means fishless. No fish in the system at all of any species. Most (if not all, someone correct me) marine parasites have a life cycle that requires fish to be present in the system to host the parasite and feed them. If there are no fish present in the system, then the parasites life cycle is broken and will eventually die off completely. See this thread
https://www.reef2reef.com/threads/fallow-periods-going-fishless.190324/
@Reefing Madness makes a good point about lymph. This disease usually starts on the tips of the fins, and can potentially works its way to the body and gills. To my knowledge so far, there is no cure for this and you must rely solely on the fishes immune system. Make sure it is fed well, no stress, and possibly supplement the food with vitamins like selcon. The fish can usually work through this disease if healthy and it may only show signs when a stress event happens, ie new fish, rework the rockscaping, or major water parameter change.
Do you have the ability to setup a QT tank?
fyi, reference this thread for a good explanation of the typical fish diseases to help with diagnosis
https://www.reef2reef.com/threads/fish-disease-index-pictorial-guide.285708/