And most likely not on the sunny side of the house.
Wash with a mix intended to remove mildew. Buy it at a big box, or make your own.
I use this mix, starting with an empty 1 gal jug:
1 qt laundry leach
1 cup laundry detergent (liquid mixes easiest)
1 cup general purpose cleaner (Simple green, Mr Clean, Spic-'N-Span,...whatever but *** NO AMMONIA***)
warm water to fill the gallon
I spray on with an ordinary garden sprayer (pump up), and highly recommend using liquids for items 2 & 3 if you intend to do the same, as the powders don't disolve as easily and can clog the screen. Brush as needed before rinsing.