From the Super Mario Wiki
The Hallway is a location in Luigi's Mansion 3. It is found in the Mezzanine on the second floor of The Last Resort. It is a hallway divided into two wings: an east wing and a west wing. The yellow Gem can be found in the east wing.
The west wing is made up of the Ballroom, Storage Room, and Dressing Room. The east wing is made up of the Restaurant, Restroom, and Entertainment Room. To get the yellow gem, both Luigi and Gooigi need to spin the pinwheels near the door to the restaurant to drop the gem from a light above.
Near the start of the game, the door to the east wing is blocked by a cart, so Luigi is only able to explore the west wing. After learning the Suction Shot, he is able to clear away the sign blocking the door to the Mezzanine in the Grand Lobby as well as the cart blocking the door.