Agree on brooklynella and this is a progressive disease which letting it go will allow it to overtake the fish. Other symptoms will be lethargic behavior, refusing to eat and heavy breathing from the mucus.
Start with a prolonged 60 minute bath of ruby rally pro then at a lower concentration in a quarantine tank. The longer the fish are exposed to the treatment, the more effective it will be at eliminating this issue.
Since a formalin solution is often not available for 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 impossible 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.
A quarantine system if you dont have one can be as easy as a second hand tank from a thrift store or as simple as a starter tank kit from Walmart which has most of the essentials