Fish fungus, while a common disease, needs to be treated early otherwise you will have a tank full of dead fish. Usually this is caused by mold in the water. One of the most common water molds is Saprolengia, which is a filamentous fungus that can appear as grey or white fluffy patches on the fish's skin. What is happening is the fungus is secreting enzymes that are used to digest nutrients, carbohydrates and proteins, namely your fish!
Never underestimate fish fungus. If you start to see these signs, you need to treat your fish right away. Letting this disease go too long, the fungus is going to start to eat the healthy tissues, causing death. It is a good idea to prevent this before it becomes a problem in the first place. Replacing water frequently cleaning the tank's ornaments and gravel, and regular filter replacement are good preventive measures. If you see the signs of fish fungus, isolate the infected fish. There are a number of topical treatments, applying a malachite green solution to clean the affected area, or a potassium permanganate paste rubbed into the lesion can help.
Long term treatments for fish fungus can include adding one to five grams of salt per liter of water. This can be done at the start, and can be done indefinitely. Another is Malachite green, .10 milligrams per liter, with three treatments over three day intervals. It is a good idea to consult with a fish specialist to find out more information on what to do if this is a significant problem. As usual, preventing this fungus in the first place is better than having to treat the fish itself.