1. Spores of moss land on your damp, Pacific NW roof.
2. Moss sprouts rhizoids and firmly establish a foot hold on your roof.
3. Moss takes in sustenance from existing damp spots, water dripping from trees, air, etc.
4. Moss reproduce like crazy in the water naturally provided by our Portland rain.
There is a way to stop moss from spreading, and to remove it from your roof. You can hire a roofer with experience in moss removal and get the issue handled. It’s likely moss will eventually grow back, but you can consult with your roofer on prevention methods to keep moss off of your home for a decent period of time.