wdc
I have seen plants at nurseries growing in very odd places. Like under the bench where they grew the year before. In fact Impatiens walleriana will grow like an orchid in trees, though it would be short lived. I have had Impatiens balfouri grow in cracks in bricks and sidewalks. Impatiens like disturbed areas where there isnt much compitition from other plants.
They will not harm your wall at all, they are too weak to do any damage.