A form of outdoor advertising with a clean is green message.Essentially it is created by power washing a message into a dirty surface such as a side-walk or an outdoor concrete wall. A specific form of green graffiti.
How legal such graffiti is depends on the laws in force for the area concerned. Some local governments require any form of advertising to require prior permission before placement. Also careful consideration needs to be given to the effect of power cleaning on the actual material being cleaned, it could well promote breaking down the exposed surface quicker than would have otherwise being the case.