You have several choices One choice is to use a chemical wash.
Chlorine based washes are very effective against mildew and other decay organisms.
However, they can lighten the wood color, and cause a roughening of the surface. If not rinsed thoroughly the residue can eventually cause a greying of the surface. A detergent must be added to remove dirt. Sodium percarbonate products use oxygen bleach to remove mildew and reportedly avoid many of the disadvantages of chlorine-based products. The third type uses oxalic acid and is recommended for cedar and redwood decks in which water-soluble resins can discolor the surface.