Decorate cake. Use spatula and black buttercream icing to ice mouth area smooth. Use tip 3 and white icing to pipe in teeth. Pat smooth. Use tip 3 and black icing to pipe dot nostrils. Use tip 16 and dark green icing to cover head with stars.
For shirt, use tip 16 and yellow and light green icing to fill in alternating color stripes, 1 in. wide, with stars.