No matter what you use to remove the mildew, as long as you DON'T mitigate the cause of the milder, it WILL keep coming back.
You need to improve 1) airflow, and 2) sunlight. BOTH of these 'things' are not friends to the mildew. Trim bushes and trees around the deck AND around the house so that air flows ALONG the walls of the house and deck. If one, stinking tree or bush 'resides' at, say, a corner of the house (or deck), it inhibits airflow. Either trim it away from the wall/deck, remove it, or MOVE it away from the house.
If you do this, you'll be cleaning the outside a lot less than you are now. My best to ya and hope this helps.
PS: Rule-of-thumb - When you plant a plant or bush or tree, first, picture how WIDE it will be at the 'size' you'd like to see it grow. Then, divide the approximate CIRCUMFERENCE by 2, and add 3' to it. THAT'S the distance from the house/deck/foundation you should plant the plant. For example, if you want to plant an Azelia Bush, where it will grow to 5' across, take 5', divide by 2' (2.5'), and add 3'. Plant the bush 5.5' from the house/deck/foundation. That way, you can walk around the bush, permit air flow, and even have some room to let it grow bigger! It may look a little funny in it's 'location' when you first plant it, but over time, as it gets bigger and bigger, it will not look as funny. Believe me, your plants do much better when they're like this. (The same rule applies when planting plants 'next' to each other. ALWAYS allow breathing room between them. And if you want, you can always let them grow a little bigger to fill in the 'gap'.) Remember, you need to allow airflow around the house/deck/foundation!!!