I have seen this happen before. As someone suggested above, if a fish get's a fungus from a pond where the ammonia is too high and it has scraped up against something and removes some of the mucous on the body of the fish, the fungus will grow and if the pond is not in balance the algae will attach itself to this fungus.
People will bring their fish to us so that we can heal them of ulcers or fungus or anything else that the fish may have.
We usually medicate the pond with salt, we have a fabulous filtration system which keeps the water quality perfect and when the fish is brought here, I will remove the fish from the pond, dry off the infected area and put polysporin on it along with other things that I add to this concoction and in no time the fungus will disappear.
I quit using test kits years ago because of the readings I would get. One pond would have healthy looking Koi, looking for something to eat and readings will say that either ammonia and nitrites are high. Another pond will have fish that are sitting at the bottom of the pond, fins clamped and looking awful yet the kit will tell me that the water is perfect and this is with up to date kits.
I believe that water tests, if they are done should be taken from the bottom of the pond where the toxins congregate. Even so, I let my fish tell me how they are feeling and if they don't look right I try to think of what be the cause of the conditions of the water and it normally has to do with a filter that needs a good backwashing and/or a good water change.
Yours Koily, Lorraine