White fluffy patches are almost always fungal in nature. Get the fish into a QT tank and treat it with an antibiotic like nitrofurazone (Furanase).
I don't see a My Tank in your profile so please post everything about your tank.
Please post all your water parameters, and tank info:
How many gallons is the tank?
How long has it been up and running with fish?
Filtration both mechanical and biological?
Describe the lighting units (type bulbs, total wattage).
What are the current water parameters? Ammonia, nitrite, nitrate, pH, salinity, temp, calcium and alkalinity. Please post numbers of results.
What else is in the tank (livestock and equipment)?
As a precaution, do an immediate 20% water change
Oh, and