I think what you are seeing is simple oxidation, but it's hard to tell with a 
picture....hint hint. Oxidation actually is a good thing on aluminum because 
it creates a protective barrier from mother nature. The down side is that it 
will come off on your hands and clothing. If you absolutely have to clean it, 
there are several really good aluminum cleaners on the market that trucking 
company's use to clean their trailers. Mix with water per the instructions, spray 
on, wait and hose off. It will come out looking brite and shiny. It won't 
kill the grass either. :)

Rummy