Remove Algae w/out Changing Color of Fence

I recently picked up a bunch of reclaimed wood fence pickets that I intend to use to build a rustic headboard. However, the pickets have some green algae on them that I want to get rid of. I know bleach will kill it, but bleach also takes away the weathered gray look. I don't want to lose the gray. Does anyone know of a product that I can just spray or bruch on that will get rid of the algae but not the gray?