As with all exterior walls, periodic cleaning may be needed to remove dirt, algae (usually green stains on the surface of the finish), or mildew (generally black stains that look like dirt) that can accumulate on the surface.  Basically, mild cleaning detergents, low water pressure, cool water, and a soft bristle brush is all you will need.  DO NOT USE solvent based cleaners; high water pressure; hot water; or wire brushes.  See Dryvit's document on Cleaning & Recoating for further information:

http://www.dryvit.com/fileshare/doc/us/description/ds152.pdf