Do you have the pond covered. A lean-to type of structure covered with polyethylene sheeting will keep the pond warmer, and even allow you to heat the pond. Melafix is questionable as a treatment, making the water smell, but no real evidence that it does any healing. It is tea tree oil, and works ???? on non aquatic animals, but in water the oil can't do much for the fish except coat the gills, which is not recommended. The best treatment would be temperature. The white that you are seeing could be fungus, which is a secondary invader, attacking wounds that would otherwise be healing. The preferred treatment for fungus would be Proform C, but it is not recommended at temperatures below 50 degrees and that may or may not help the wounds heal, just kill the fungus.
Zone 7 A/B
Keep your words sweet. You never know when you may have to eat them.
Richard