This discoloration. I realize your region and also your humidity and weather.
Mary Jo, the first thing I would ask of you: do you have adequate breathing room and areation for your roof? Do you have low hanging branches that hinder the sun from drying out the roof after a rain? Is there a canopy of branches that create an umbrella if you will that lets water and water vapor sit on the roof for long times thus creating mildew, and stains?
If the answer to these questions are yes, then I would first try to let the roof breath more. This may fix your problems, and there to, may not.
I would be soo careful about powerwashing the roof. If the person/persons are not professional and aware of the damage that may result from irresponsible cleaning, you could have more problems than you do have now.
Also, any isolated cleaning of the roof will also create a "clean" area and will be very conspicous from the other parts of the roof. If you clean one spot, I would just clean the whole roof area.
Its like spot cleaning an area of carpet, one area is nice and clean and the rest is one big total carpet stain.
Jay, the moderator, lives in an area similar to your environment and he may have some other ideas. I learned from him about the opening up of the roof for drying out.
This situation needs alot of research and thinking out.