The best method to remove mold and mildew from a deck is to clean it off using a steam-power sprayer. The sprayer is capable of applying heated water at a high pressure that washes away the mold and mildew, along with the roots that sink down into the deck's surface.
The ideal water temperature sprayed onto the deck is 250 degrees Fahrenheit or more. After removing the mold or mildew, removal of any stains left behind requires cleaning the deck with a sodium percarbonate product placed directly onto the stains and left in place for 6 hours before removal by rinsing off the deck. Keep the sodium percarbonate product damp while it works on the stained area.