Question

how do sapling sometimes grow on wall?

Answer 1

some trees is hemi-epiphyte in its native habitat.the seeds germinate and grow as an epiphyte on other trees and when the host tree dies they established in soil.an epiphyte is a plant which grows on other plants or other objects like wall.
hope u understand.

Answer 2

Actually it mostly happen in rainy season that walls appears green.Those are not saplings becoz they remain like that only throughout the season.It is actually are green algae which appears green when water is given on them