Reverse Graffiti

From what I learned on Wikipedia.org 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. It is often done by removing dirt/dust with the fingertip(s) from windows or other dirty surfaces, such as writing 'wash me' on a dirty vehicle. Others, such as artist Moose, use a cloth or a high power washer to remove dirt on a larger scale.

It is a fat marker to fill with soapy water that will let you draw in urban dust.