I have algae growing on my deck, flower pots, outdoor furniture and siding!
Using a pressure washer would be your best option for removing algae on your siding and deck, etc. You can rent a pressure washer from a hardware or home improvement store, or hire a service to do the washing for you. Pressure washing with a strong stream of plain water would be the most environmentally friendly approach -- it will remove a lot of the algae and surface dirt, but probably not all of it. For a thorough cleaning, use a biodegradable detergent (not bleach). Read and follow the product application instructions.