Lets get things started with a first things first. The most important thing with fish is the water. We like to see the numbers, preferably from liquid drop type tests instead of dip strips which can be real iffy. The numbers we are interested in are ammonia, nitrite, nitrate, pH morning and evening (High Range Test as it should probably be over 8), KH, water temperature, gallons of water in the pond, filtration system (type and size), does the pond have rocks on the bottom or not.
The grey film is a thickened slime coat, and this is a defense to irritation. Irritation can be coming from ammonia in the water, large swings in pH, parasites, or possibly other issues, but since the water is so important for future healing as well as current health, we like to start there.
John at Hanover Koi is good, but some of his treatments are different than traditional. The manufacturer of Proform C states that it should not be used with salt, but John has used it with salt at some pretty high values. Salt should never be used at values much higher than 0.9% as it will desicate the fish. Because salt is an irritant, it can cause thickened slime coats, which on the gills will interfere with oxygen transfer. If as you state you are up to about 1%, then it would be a good idea to do a 50% water change to reduce it to a reasonable value.
In addition to the Proform C, I would order Fluke M preferably, or Prazi for treating for flukes. Proform C will take care of the other microscopic parasites.
Zone 7 A/B
Keep your words sweet. You never know when you may have to eat them.
Richard