Greenie

Greenies are average sized green ghosts that first appear in the Gloomy Manor of Luigi's Mansion: Dark Moon. They are the first ghosts that Luigi encounters in the game. They were originally helping Professor Elvin Gadd in his experiments with ghosts, but once the Dark Moon shattered they turned on him. Greenies have 10 HP, which makes them technically the weakest ghosts encountered but to compensate for that they are encountered in groups of 2-5, sometimes as support enemies when fighting stronger ghosts.

E. Gadd's Vault Description
"They like to tinker with things they shouldn't and wield everyday objects as weapons. First, find a way to disarm them, then stun them with your flashlight when the time is right."

The official bio on the website quotes, as well as the above: "They may be the mildest of spooks you'll encounter but they can still cause quite a bit of trouble for our intrepid ghost hunter."

Abilities
Greenies have several different abilities that are introduced throughout the Campaign:

Greenies attack with a short ranged punch, similar to a Gold Ghost which deals 20 damage to Luigi if successful. Greenies are able to equip themselves with various items from the surroundings to increase their offensive and defensive powers. They can, for example; equip themselves with a rolling pin or a shovel to increase their attack power, or they can equip themselves with sunglasses or a helmet to block the Strobulb's effect on them, enhancing their defensive power. Greenies are also capable of disguising themselves as mummies or knights to further elevate their strength, forcing Luigi to employ special tactics to defeat them. In the Old Clockworks, they introduce the ability to create mini-tornadoes in the sand which have a considerable attraction force and will damage Luigi if they pull him in. They can also use Paranormal Shields to grant themselves immunity to the Poltergust 5000 (in the form of infinite healthpoints) which requires Luigi to break their shields before they become vulnerable to capturing.

Trivia

 * The description for the Greenie in E. Gadd's vault says to stun them with your flashlight, but this is untrue. You actually use the Strobulb.
 * This is most likely a reference to the flashlight in the first game.