Your fish has most likely contracted what is known as "ick". Go to a pet store that sells fish. They have medicine that you can add to the water. Tell them at the pet store what is happening to your fish in case it should be something else. Good luck.
The white stuff on your fish could be ich, which is mostly caused by poor tank conditions. You need to check the tank water conditions, where you test the level of the ammonia and nitratre. Poor conditions can cause fungus on the fish. It would be best that you get a vet to examine your fish and offer treatment.