You need to presume something about the substance before calling the police - absent expertise or testing equipment, you're suspicions are just that - baseless suspicion.

The police are being called without any clear evidence that wrongdoing is afoot - therefore your actions are illegal and the courts should be quashing this evidence if/when it gets to court.

You are trained to identify a gun - you are not trained to identify or verify the contents of a bag containing white powder or a grassy substance.