The Mist That Blocks Perception of Supernatural Beings

Sometimes, sometimes just weird, weird, explicable stuff happens to him. Like he goes to an art museum on a field trip and a teacher like turns into a nightmare beast and tries to kill him. There's a thing called mist that blocks their perception of supernatural beings. So Fia says, gaslight gate keep Grover. Which is a thing that Grover is kind of doing to Percy, where he's not really letting on what is going on.