Here in the northwest, most information is about removing moss and lichens.  Algae should respond to a bit less effort.  For instance, I’ve found that periodic vigorous sweeping of my sunny concrete patio will remove algae while still wet and, then, I repeat the process as needed through our rainy season.   

Some of the methods used against moss will work against the algae. Consider a power washer, making certain to decrease the pressure to avoid damaging the wood. Or apply a commercial solution made especially for use against moss and, then, thoroughly rinsing the deck. Commercial chlorine bleach can be strong smelling and somewhat risky to handle whereas oxygen-bleaches are milder but effective against algae. Good sources of mild but effective products are paint stores and large garden centers.

Still more ideas: ”Methods of moss removal”  http://bryophytes.science.oregonstate.edu/page18.htm; “Outdoor cleaning solutions” http://extension.usu.edu/files/publications/factsheet/pub__3096233.pdf ; and “Siding and other exterior surfaces” https://fyi.uwex.edu/house/about-the-house/siding-and-other-exterior-surfaces/

During the cleaning, protect nearby delicate, fragile, and/or treasured garden plants with a light cover.