Apparently, Mob couldn't stop the Dash Granny because she wasn't a ghost but a real person? But Mob can exert his psychic powers over ghosts and humans, so why couldn't Mob stop Dash Granny?
From the anime, we know that he won't use his power against humans. 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 humans or ghosts. So, when he tries to use his psychic power to manipulate ghosts against Dash Granny, it seems that it doesn't have any effect. That's why he knows that Dash Granny is not a ghost.