Place popcorn in a large bowl. Pick out and discard all unpopped kernels.
Melt butter and caramel sauce in a large pot over medium-low heat. Add marshmallows and gently heat, stirring, until melted and smooth. Pour over popcorn and stir until well coated; let cool slightly. Grease your hands and shape a handful of popcorn into a ghost shape, pressing firmly to shape. Continue with remaining popcorn, greasing your hand more as needed to minimize sticking. Let cool completely.
Melt candy melts according to package instructions. Use a spoon to spread melted candy over popcorn ghost in a thin layer. Gently press eyes onto the ghost and let coating harden. Store, covered, at room temperature for up to 3 days.