Any surface that stays moist will grow mold. I suspect the moldy vinyl siding gets little or no direct sunlight and the wind does not freely circulate around these moldy spots. This is like moss growing on the north side of trees in a forest, the sunlight never shines on the north of the tree.

I suggest a light solution of 3 quarts bleach, 4 gallons water and 1 quart of either Jomax or Simple Green. Wash the siding with detergent or a deck cleaner, rinse with a hose, then brush on the water and bleach solution with a push broom, or broom head, on a long paint pole. Let the bleach liquid dry on the siding to leave a bleach residue behind. It will wash off with the first rain but will protect a bit longer.

Use bleach in one gallon of cold water to rinse on the deck.

Regular cleaning of the siding, just like washing your car, helps keep the dust and pollen from building up on the surface. This dust and pollen residue holds the moisture and promotes mold and mildew growth. Regular washing of the siding keep the “green” at bay. Be careful with the bleach and Jomax mixture. Wear rubber gloves, eye protection, protect nearby vegetation from the bleach rinse and be aware that deep color TREX pigments can be affected and “lightened” by the bleach.