Home centers (like Home Depot) carry a product just for this- you can use it without a pressure washer. You use a large push broom to give it a good cleaning. Ask in the paint department. (I'm an HD junkie!)
I have used the line of strippers / cleaners and Brightener from Penofin. The cleaner is a dry mix that you mix with water onsite, application via a pump srayer. To help I use a 8 to 10" wide "deck scrubber", rinse is with the garden hose.
Are you sure it's mold? It sounds like moss. Maybe this site can help you http://www.ehow.com/way_5921100_diy-moss-removal-wood-deck.html
A lot of this is actually algae...believe it or not. The cleaning products I mention above work on all type of stuff. The beauty of them is they are pH balanced. Too many people use bleach which is very alkaline and messes up your wood.
Spray the "mold" with weed killer. I read that this is actually a plant and the weed killer worked on places on a walk and also on my stone walls.