Green mould

Bit tongue in cheek really - but why does my van grow green algae, my car stays out all year round and nothing sticks to it (except the dreaded seagull presents).

I always keep the van clean and scrub these offending areas with a toothbrush, have tried a weak bleach solution to kill the spores but nothing works. Leave it for a couple of weeks and you can guarantee that the green stuff will appear somewhere. Do others have the same problem?