Mental Homeless patient outside on my street?

ok so i never encountered or seen this guy but my parents have. He runs up to people and makes queer faces at them like as if he is angry. He stands in the middle of the road and pulls a wrestler move in the middle of the street ( flexing his arms in the middle of the road when cars are coming) and today my mom saw him walking in the middle of the street in the rain with his head down. I know this sounds ridiculous but this is real. Ok so i said that we should call 311 but my parents said he didn't do anything to harm people yet but he's a mental patient i mean which person in their right mind would do that stuff? Its a matter of time till he comes and attacks someone. What should we do? And if he needs help he shouldn't be out in the open like this