Place the potato pieces in a large pot and cover with water. Bring to a boil over medium-high heat and cook for 15 minutes until the potatoes are fork-tender.
Drain the potatoes and return them to the pot. Add the milk, butter, salt, and black pepper.
Mash the mixture with a potato masher until smooth and creamy.
Divide the mashed potatoes evenly among four bowls.
Use a spoon to scoop small portions of mashed potatoes and shape them into ghost forms on top of each bowl.
Press two crackers upright into the mashed potatoes in each bowl to form gravestones. Sprinkle the sliced green onions around the ghosts and crackers.