I have had success with doing a 10 minute saltwater dip in Hydroplex. It kills the parasites with low stress and gives them quick relief. You would still need to treat all of your fish in a hospital tank. If you can't get hold of formalin, Copper Power or Cupramine will also kill the bugs and can be purchased at your LFS.
Copper does not kill brooklynella.
For future reference, I have used formalin baths and baths in Ruby Reef Rally Pro quite effectively. It works much better in a bath than in a display IMO, and has no risk to corals or inverts doing the bath method. Then all fish go into quarantine for 6 weeks while the display sits fallow.
To the OP. You have brooklynella in your display. All fish need to be pulled and treated, even if asymptomatic, and the display needs to sit fish free for 6 weeks to successfully eradicate this parasite. It is easy to introduce to a system, and fish can be asymptomatic carriers. My wife added a pair of firefish to her evo with her clownfish pair a few months ago and within a week, both clowns started getting a mucous slough like that, just not quite so bad, and had depressed appetite. Firefish were never affected. Treated all fish with the ruby reef rally pro bath, then moved them to a sterile quarantine and left the display fallow for 6 weeks. Fish are back to 100%, and back in their tank now after the fallow was done.
For a Ruby Reef Rally Pro bath, it is 1 teaspoon (5mL) per gallon of aerated saltwater at the same temp and salinity that the fish came from, for 90 minutes. They are then transferred into a sterile quarantine, with nothing moved from the display other than the now treated fish. Use a bottled bacteria like BioSpira or Microbacter7 to "instantly cycle" the quarantine.
