I can't tell much from the pictures. The color doesn't look to be anything of import, more like color change. If the fish had an infection, I would expect to see fiery red coloring, not grayish white. If the fish is acting normal, I would do nothing in the way of treatments, but I would like to see you do routine testing, preferably with drop type test kits, like API for ammonia, nitrite, nitrate, high range pH, and KH, as the numbers tell so much about what is going on with the water, which is the environment that the fish has to live in. You didn't mention frequency of water changes, which should be around 10% or more per week. I know that at this time of year, you will probably have frozen hoses that prevent some water changes, but there are also many days that the hoses will be thawed.
Can you also describe your pond and filtration for us.
Zone 7 A/B
Keep your words sweet. You never know when you may have to eat them.
Richard