Hell of a lot better than gang sign. I’d say leave it. You paint it over, you’ll probably get something worse.
I went to college in NYC in the '50s and I distinctly remember entire themed subway trains, my favorite one a beautiful serpent snaking down the El. Unfortunately, for every street artist, there are a thousand taggers whose sole contribution to art is "JULIO" in ugly da-glo, or worse, gang-graffitti.