reverse graffiti
n.— «Paul Curtis aka Moose is no regular graffiti artist. In fact, he’s the reverse-graffiti artist. He created his street art by cleaning the dirt and grime off of surfaces! Authorities are baffled: is selective cleaning a crime?» —“Soap Not Spray Can: Reverse Graffiti Art” Neatorama Sept. 13, 2006. (source: Double-Tongued Dictionary)