I was able to expand and freeze the video. Looks like brooklynella which is often called clown disease although it can affect other fish. Especially noticeable on the percula is the amount slime on its body along with thick mucus on its body also noticeable on the fish. This mucus generally starts at the facial area as well as gills and spreads across the body producing lesions as it progresses often confused with ich and can turn into secondary bacteria. Other symptoms will be lethargic behavior, refusing to eat and heavy breathing from the mucus.
Typical treatment is a formalin solution is mixed with in a separate container with either fresh or saltwater. Start with a quick dip in the formalin at a higher concentration then performing treatment in a prolonged bath of formalin base at a lower concentration in a quarantine tank. The longer the fish are exposed to the formalin treatment the more effective it will be at eliminating this issue.
If a formalin solution is not available for immediate use, temporary relief can be achieved by giving the fish a FW bath or dip in water same temperature as display tank. Even though this treatment will not cure the disease, it can help to remove some of the parasites, as well as reduce the amount of mucus in the gills to assist with respiration problems.
Treatment is best done in a QT tank using either quick cure (more effective but now harder to find) or Ruby Rally Pro. Ruby takes a little longer and initial treatment generally takes 2-3 days to really start going to work.
With the advanced stage of this- I recommend immediate quarantine of all inhabitants and leaving display without fish for 4-6 weeks.
