The only way to prevent mold growth on your house is to try to allow the natural breezes to reach the siding and trim. Bushes should be away from the house, etc. Adding additional mildewcide to the final coat of paint is another option. This will give you a couple more years protection and prevention.
The type of paint also affects mold growth. Use a semi-gloss acrylic. Mold doesn't grow as quickly on this paint and it washes off easier.
The mold needs removing before it gets too bad. Use a solution of 1 part bleach to 3 parts water. Applying with a garden sprayer is a good option. There are commercial mold removal products on the market that work. These products can be injected into a pressure washer for fast removal.