Should i call the cops if a girl flashes or moons me?

Yes: This isn't a forum about law or our complicated world with changing gender roles, but i'd urge you to report it. They would do it if you did that to them. The girl may be suffering from mental illness, be on dangerous drugs, or be trying to manipulate you dangerously. In any case, she is on the way to trouble. Good luck.

Not really: This can be either an insult or a flirtation. It certainly is generally benign and if it bothers you look away or respond to the come on. I think you would be embarrassed by the response from officers of the law called to investigate such an event.