Is that legal marijuana harmful?

Yes. It is not really different from any other marijuana. Alcohol and cigatettes are legal and cause horrible health problems. Whatever views you may have on drug policy, it has negative effects when abused, just like anything you may put in your body.
In a way. Legal or illegal marijuana can be harmful. Although, there is no overdose of marijuana described, it continues to be a common cause of violent behavior in users and in some causing psychosis. It can impair enough to provoke accidents, including car wrecks. Long term use has been linked to failing memory in students and early onset dementia.