Lots of greenish mould growths on local fences around here too. Probably a good wash down with a pressure washer will clean it up.
I cleaned the PVC on my Conservatory last week and there was a huge amount of green mould/algae on the underside of the guttering which I haven't had before. Cleaned it all with PVC cleaner followed by soapy water and a final hose down. Its sparkling now !