Reverse graffiti, also known as clean tagging, dust tagging, grime writing, green graffiti or clean advertising, is a method of creating temporary or semi permanent images on walls or other surfaces by removing dirt from a surface.

Because reverse graffiti is temporary, may be biodegradable, and generally uses no hard materials such as ink, paper, or harsh chemicals, and because it generally does not use electricity to back or front light, reverse graffiti can be an environmentally friendly way of advertising.

There have been several instances of authorities attempting to prosecute those performing this form of advertising, but prosecution has been difficult due to the temporary and non-destructive nature of the practice.

Images via scribol. From top, Scott Wade, Moose, Moose (again).
Quoted text via Wikipedia.