Apparently Mob couldnt stop the dash granny because she wasnt a ghost but a real person? But Mob can exert his psychic powers over ghosts and humans, then why Mob couldnt stop the dash granny?
From the anime we know that, he won't use his power against human. There is one reason why he couldn't stop dash granny.
This is speculation. The other reason I can think of is because, there is a different type of psychic power to be used against human or ghost. So, when he try to use his psychic power to manipulate ghost against dash granny, it seems that it doesn't have any effect. That's why he know that dash granny is not a ghost.