I think you have a simple solution. I think you need to mix water with bleach, there is a proper ratio, and than take a pair of vinyl/rubber gloves on hand, maybe as many as 2 or 3 various size brushes, and brush that mildew black away. Somehow, I think you are going to wind up with a beautiful white stone that mimics your window concrete? stone? framing, etc. etc. It is not ugly, it just needs to be cleaned.--- When we had our stone patio built, I asked the stone mason what to clean the stone with. His answer, "White vinegar and water, mixed half and half." You can use large sponge (Lowes has them in the paint section). You can also use a stiff bristled brush with the vinegar/water mixture. ------ In Rome they are cleaning the great arena with water and tooth brushes. They expect to be at it for 10 years. I would start with an assortment of brushes (soft to stiff) and a bucket of water with Dawn dish soap. Start with the softer brushes and work up to the stiffer brushes to find the best one. ------ wash with Awesome Orange mixed into cleaning solution --- This is just mildew. Spraying with half ordinary bleach and half water will take care of it and avoid power-spraying water under the stone veneer. As a bonus, ordinary bleach, sodium hypochlorite 5%, breaks down into NaCl, table salt, and H2O, water, nontoxic to your environment. -----some recommend a power washer, on the lowest setting only so you don't wash the mortar out. Different washers equal different low pressure. I think your variety and sizes of brushes with bleach and water, is the best bet, but the others are there. Make a one cup mix of the various options and see which works best in a small area. It may be that one need one blend for the heavier dark areas and another blend for the lighter areas. I would do this when there is just a bit of wind, so the odors blow away from you. My friend bought 3, 6, 9, stone off a demolished house built in the early 50's, & along the foundation the stone was very dark, as well as up the steps to the house, and using a brush with bleach, made it sparkle like diamonds. They wrapped it in plastic wrap after it dried and had stacked it on pallets. It will be the exterior of their new home.