What is it?
Where does it come from?
How do you get rid of it?
This is called oxidation which can be caused by over sun exposure to your home or leaving an oxidizing chemical (bleach) to long on your home before rinsing off (average dwell time is 10-15 minutes). The best way to check for this is to run your hand along the side of your house and see if it comes off on your hand.
The best way to get rid of this is with a good brush, elbow grease, and cleansol bc.