Nope it's legal. §303 StGB:

Whoever, without being authorised to do so, substantially and permanently alters the appearance of an object belonging to another [...]

Projections are far from permanent. What I'm blanking on right now is whether cleaning something is captured under that wording, I vaguely remember them discussing changing it, dunno if they have. If a wall is dirty enough you can make a great graffito with nothing but a pressure washer.