CHECK THIS OUT

As a man who both paints on walls and has tried to grow moss for years, I find this intriguing. 

The only problem I see is that there dozens of different types of moss, and it doesn't tell you which to use. Also, I know as a fact that you would have to paint that stuff on the shady side of the building, since moss loathes sunshine.
Plus I have no idea what water-retention gardening gel is.