How do I prevent algae from growing on my vinyl siding?

I found this article that might be helpful for you - http://www.silive.com/homegarden/index.ssf/2013/05/the_dirty_truth_about_vinyl_si.html, good luck

You can purchase two products that will clean and then prevent algae/mildew from growing on your vinyl siding. Both of these products can be purchased from Lowes. The first product is called "JoMax" which will clean your siding (https://www.lowes.com/pd/JOMAX-128-fl-oz-Liquid-Mold-Remover/3099065). You will need to cut it with bleach and water per instructions on the container and apply it with a garden sprayer. Once applied, let it sit for ten or fifteen minutes and then rinse it off with a garden hose. After your siding is clean, let it dry completely. The second product is called "GhostShield" which will protect your siding from dirt and moisture (https://www.lowes.com/pd/Ghostshield-Clear-Flat-Concentrated-Waterproofer-Actual-Net-Contents-128-fl-oz/999911189). There are several waterproofing products available so if you want one with a gloss or texture, search the Lowes website for "Water Proofing". The water proofing products also come in different colors in case you are not looking for a clear finish. Just like any product you use, apply the water proofer to a small area before you begin to see if you like it. You can also use this stuff on bricks, cement, wood and just about any other surface that you can imagine.

I hope this helps...