Spray loose debris off the Polywood with a garden hose.
Mix 1 tblsp. of dish soap into a bucket of hot water.
Put on protective gloves (rubber or vinyl) to spare your hands and douse a sponge in the hot soapy water. Rub the sponge over the Polywood, paying extra attention to moldy areas.
Wet a nylon scrubber in the soapy water and scrub the remaining stains the mold may have left behind. Use a nylon-bristled brush, instead of a scrubber, to get dirt or mold from creases (you can use a toothbrush for small areas).
Follow the stain-removal advice from the manufacturer for any remaining stains: mix 1 part bleach to 2 parts water in a bowl. Wet a sponge in this solution and rub it over the stain. According to polywoodinc.com, this solution will not affect the color of the Polywood.
Rinse the Polywood with clean water and dry with a towel.