Line a 13 x 9-inch pan with parchment paper. Spray evenly with oil. Pop popcorn according to package directions and remove any unpopped kernels.
Melt butter in a large saucepan over low heat. Add marshmallows and stir until completely melted. Add 20-30 drops grass green food coloring, or enough to reach your desired color. Stir well to combine. Remove from heat.
Add popped popcorn to the marshmallow mixture. Stir until well coated.
Using parchment paper sprayed with a little oil, evenly press mixture into the prepared pan. Cool. Cut into monster shapes and decorate. Best if served the same day.