This is an NPC, or a non-player character.
Ghosts spawn in a world which has not been visited for a long time. They can spawn in any world, with the exception of worlds with Ghost Charms placed in them. A limit of 20 ghosts can be spawned in a world.
A Ghost flies around mindlessly. You can see ghost by using Spectral Goggles or a Synthetic Oculum. However, it can move towards a Growtopian equipping a Neutron Gun and a Neutron Pack whose punch is in its way. Ghosts can slime players upon colliding with them unless the player is wearing Rubber Boots. Slimed players move slower and jump less higher than normal. When a Ghost comes in contact with the rings of a placed Ghost Jar, the Ghost Jar and the Ghost will disappear, and a Ghost-In-A-Jar will appear in your backpack.. The Ghost-In-A-Jar can be used for items from the Spirit Storage Unit, when it explodes. Also, if it explodes, any "unused" ghosts will be released.