http://www.youtube.com/watch?v=4wSq0YGHQWE
I don't understand: do they use acid to etch the sidewalk? I don't get the "green" part...
Think "green" as in eco-friendly.
They use an ordinary power washer and only the bits cut out of the stencil get washed. The cleaned concrete is lighter in color.
The ad will fade over time as the cleaned bits get dirty again. If someone wanted to erase it they could just wash the whole thing.
Now this is green graffiti
http://www.thepoke.co.uk/2012/03/01/moss-graffiti-for-beginners/