Graffiti is generally given a negative connotation. It’s a sign of urban decay, lax security, some douche with a spray can and too much free time.
Stuff that ventures beyond someone just writing their name on a bridge can approach the slightly haughty term ‘urban art’ when it’s conveying a message, like some of the best works by Banksy.
But do you call it when graffiti artists paint by removing dirt and grime from formerly clean areas? Reverse graffiti? Anti-painting? An eco-statement? “Addition by subtraction”?
This particular artist’s work forced the Sao Paulo city government to clean the soot from every tunnel in the city to prevent further ‘vandalism.’ So cool lookin’ and effective.