Hallway (Hotel Shops)
From the Super Mario Wiki
The Hallway is a location in Luigi's Mansion 3. It is found on the second level of the Hotel Shops. It is next to the Mall Lobby and south of the Security Office and Lost and Found. The blue gem can be found here.
When Luigi first enters the mall lobby, he sees Kruller patrolling the area. He inspects the vending machine in the hallway to find a mouse in it which startles him. He then enters the security office, dropping the diamond key needed to unlock the cash register in the Accessories.
There is a vending machine in the hallway. There is also a crack in the window blinds Luigi can peek through to see Kruller at his desk holding the second floor elevator button, taunting him with it. To get the blue gem, Luigi needs to flash the vending machine with the Strobulb several times, getting the following each time: a coin, a Bill, a Gold Bar, a mouse, five mice, and at last the gem.