Best way is to power wash it. You can make your own cleaner here:
70% water, 30% white vinegar makes a great all-purpose vinyl siding cleaner that removes light mold and mildew stains. For a stronger solution, mix together one-third cup powdered laundry detergent, two-thirds cup powdered household cleaner, one quart liquid laundry bleach and one gallon of water.
oxyclean as the cleaner, less toxic than bleach.
Bill....We use a pressure washer and Krud Kutter from Lowe's.
Use green Clorox bottle. Works like a charm every time. I use it with baby wipes sometimes... I am weird...
Hope it helps! Bushra