Reverse Graffiti
According to wikipedia the definition of reverse graffiti is:
Reverse graffiti also known as clean tagging, dust tagging or grime writing, is a method of removing dirt from a surface while creating graffiti on walls or other surfaces. It is usually done by removing dirt/dust with the fingertip(s) from windows or other dirty surfaces.
Check out this great article at inhabitat.
UPDATE: Watch a video of the process hereÂ
[...] Mossenger is a brilliant take on graffiti by Anna Garforth which focusses more on natural non-destructive elements (in this case moss) with which to make her statement. Kind of reminds me of Reverse Graffiti. [...]