is this graffiti

I know this is trivial but i would appreciate any help. I own a pressure washing business. Typically when I'm in a neighborhood, I will pressure wash my co. name and phone in in the street gutters with a stencil. Recently I stenciled one on the concrete where a group of mailboxes was sitting. I got a call from the postmaster. I am not painting; I am removing dirt and the clean spot that remains spells my name and number. Am I really breaking the law according to the definition of graffiti? I looked up some federal codes under the post office section and didn't see anything that looked like it applied to me. I would hate to have to remove all the stencils that I've done. Wouldn't that be the same thing? I would just be cleaning a bigger spot of concrete (without my name and number of course).

Thx for any help.
blackjack