All exposed timber, including vertical cladding, is eligible for algae/lichen contamination. After an initial period of exposure, the natural chemicals imbedded in the cellulose degrade and are washed away. The surface becomes grey when bare cellulose needles are exposed. This surface becomes a natural habitat for algae, then later moss and lichen.
Oak cladding colonised by a black lichen
The only realistic way to maintain the grey surface on a naturally bleached wood surface is to sterilize it as and when the green sheen appears. It is also a good approach to wood preservation. AlgoClear will perform this without damage to the environment.
Warning: If the decking has been allowed to become too slippery, a brushing may be necessary prior to spraying. AlgoClear will, however, be sufficient to maintain a safe surface.