Place the softened butter in a mixing bowl and beat it until smooth.
Add the powdered sugar a little at a time while beating on low speed.
Pour in the milk and continue beating until the frosting reaches a thick, spreadable texture.
Add several drops of green food coloring and mix until the frosting turns a bright, even green.
Spread or pipe a thick layer of the green frosting over each cupcake so it looks like dripping slime.
Press two candy eyeballs into the frosting on each cupcake.
Sprinkle green sanding sugar over the top of the frosting for extra sparkle.