It's usually lichens and mosses, and in the British climate very difficult to prevent. A good scrub out and a pressure wash at each end of the season is the only thing we find we can do; doesn't prevent it, but at least keeps it under control.
I take it you park your van under trees!! If pos either put a plastic sheet over the roof or move to a better position.
Fairy liquid will do it, & as said a power wash.