No clue of city policy, but apparently there is a city graffiti task force, if you call 311, they send you a form to fill out. I have had no success calling 311, but that's what's been passed on.
I haven't had a chance to look at it but I have experience removing graffiti. It comes of easier if it is pressure washed before it becomes hard. I own a pressure washer and would help if needed but I'm not sure of policy of city propriety. Paul ps. of no charge.