Insert a lollipop stick or dowel into the top of 6 apples. Microwave 3 cups white candy melts in 30-second intervals, stirring, until melted. Dip each apple into the white candy and let the excess drip off. Transfer to a parchment-lined baking sheet; let set before decorating unless otherwise directed.
Mummy: Attach 2 candy eyeballs to a white candy-coated apple using more candy melts. Microwave 2 marshmallows until puffed, 10 to 15 seconds. Stir until smooth; let cool slightly. Using a spoon, scoop up the marshmallow (it will stretch into a thread) and wrap it around the apple.
Spiderweb: Microwave black candy melts until melted; transfer to a small resealable plastic bag. Snip a corner. While a white candy-coated apple is still wet, pipe a spiral onto one side with the melted black candy. Use a skewer to gently drag lines out from the center to create a web pattern. Let set.
Cat: Snip a marshmallow in half diagonally, then trim to make 2 triangles. Press the cut sides in black sanding sugar. Attach to a white candy-coated apple using more candy melts, then attach 2 green candies for eyes. Pipe lines with black icing.
Skeleton: Roll mini marshmallows between your thumb and index finger to make them into a bone shape. Attach the marshmallows to a white candy-coated apple in a hand formation using more candy melts.
Werewolf: Drag a skewer across a white candy-coated apple in 3 spots to look like scratches. Fill the scratches with red gel icing.
Ghost: Make a puddle of melted white candy melts on parchment; set a white candy-coated apple on the puddle while still wet. Let set, then pipe eyes and a mouth with black gel icing.