From the Super Mario Wiki
The Waiters are servant ghosts from Luigi's Mansion. They are somewhat skinny white ghosts that can carry a plate of food in each hand.
Luigi runs into two Waiters in the Dining Room as they are serving Mr. Luggs. In order to capture Mr. Luggs, Luigi must provoke the gluttonous ghost into attacking by cutting off his food supply. To do so, Luigi must capture the Waiters to stop them from delivering food. These ghosts are easy to defeat, as they possess no attacks. Waiters only have ten HP. If Luigi manages to take all of Mr. Luggs's food away before the waiters come in and refill it, they will flee in terror as Mr. Luggs goes into a rage.