I just discovered this new art form and I think it's pretty fabulous. It's reminiscent of reductive monotyping and it also draws attention to environmental concerns. The "graffiti" is created by cleaning dirty walls in strategic ways to create art reductively.
Though the police are seen harassing these artists in the video, I can't see anything illegal about cleaning a wall. Even so, it seems the powers-that-be saw fit to wash off the work before it even got an extended viewing. Luckily there was a videographer on hand to document the piece before it was washed away.
And here is a link to the Reverse Graffiti Project which shows another video using stencils and a powerwasher: http://www.reversegraffitiproject.com/. This time, the artist got obvious sponsorship. Again, the technique is pretty cool, though I find the prominent product advertising, unfortunate.