It always amazes me the GUNK that sticks to our cement and brick  walkway slabs in 1 year.  Every year I spend a full day pressure-washing all these front & back, and also my pool deck which has composite decking but still accumulates dirt that needs to be washed off.   This year I had a little drawing fun before cleaning them all off.   This isn't chalk on cement;  the dark parts are slime, the bright parts what it looks like after the pressure wash water cleans it off.