Dressing Room (Mezzanine)
From the Super Mario Wiki
The Dressing Room is a location in Luigi's Mansion 3. It is found in the Mezzanine on the second floor of The Last Resort. It is east of the Storage Room and north of the Ballroom. The green Gem can be found here.
When Luigi discovers Professor E. Gadd trapped in a painting in the Ballroom, he needs to retrieve the Dark-Light Device from the Storage Room. However, the Storage Room is locked, so Luigi needs to get a Key hidden in the Dressing Room. To get this key, he needs to locate the mirror in the northeast corner of the room and flash the green light near the southeast corner with the Strobulb. After doing so, the key pops out of a compartment on the east wall. When Luigi exits the Dressing Room, he is spotted by two Goobs hanging up wanted posters of him. He must then defeat these ghosts to continue.
To get the green gem found in the Dressing Room, Luigi needs to pull off the purple coat hanging on the coat rack on the left side of the room, revealing a safe he needs to flash the reveal the gem.
There is a bag of Bills on the left drawer. Luigi can also suck up the clothes on the coat rack to get bills. There are even more dollars in the overhead lights, which Luigi can bring down by vacuuming. There is more money in the left and right drawers. There are also several invisible objects in the Dressing Room Luigi can reveal after getting the Dark-Light Device. There is a treasure chest in the southwest corner and a bin in the northeast corner. There is also a chair opposite the right drawer which reveals a Golden Goob when turned visible. There is an achievement called "Found all haunted objects in the 2F dressing room". To get this, Luigi must reveal all of the invisible objects in the room.