Its just the standing water and the time in the sun that let the algae grow..
you will need to kill the algae with a mild solution of water and household bleach , I use a toothbrush for the crevices ...
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tastes funny inthe morning tho '
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but you still need to use elbow grease and something to get it off the paint, like suggested previously.
it doesn't come back again for along while.
C