Yep. EditorRUS is right, the mop doesn’t do it. If the player mops the tile he’s standing on the following message is displayed:
“You mop yourself up.”
“The universe implodes and reforms around you.”
But the graffiti is stays there.
And trying to mop the graffiti on an adjacent tile gives the “There’s nothing to mop there.” message.
I guess the graffiti is not moppable since its not considered a spill. Currently, only spills like blood splatter or spilled water can be mopped. Spraying graffiti using spray can or writing using permanent marker doesn’t make any difference.
add_msg(_("You mop up the spill."));
I think its a bug, and if it is, then it should be fixed.