Need pics

Sounds to me as if wood is rotting somewhere as those growths are there because of moisture.
How are the posts set into the ground?
That's where I'd look first.
And how were the posts covered with vinyl? That seems another area ripe for trapped moisture.
You need to figure out where moisture is getting at wood and stop it from happening.

If you have a wooden porch or fence, the chances of weeds or plants such as vines to flourish and grow on them as well can be high especially if you do not regularly clean this area. As long as the plants or what grows on your porch do not damage it or affect the sturdiness of the structure, consider it as one of the unique features of your home addition.

"Maintainence free" vinyl will support the growth of algae, fungus, mushrooms and other flora. Cleaning with a 10% bleach solution after removing whatever growth you can by hand is the only real approach. It will be ongoing, especially in warm, wet summers such as much of the country experienced this year.