In advance: Make 2 coral clusters and 1 sea plant group for each cupcake using royal icing. Cover board with waxed paper and pipe tip 14 coral branches in random shapes, approximately 1 1/2 x 1 1/2 in. wide. For sea plants, pipe tip 352 leaves in heights up to 2 in. Make extras to allow for breakage; let dry overnight.
Spatula ice cupcakes in buttercream, using swirl motion. Position topper and insert royal icing decorations.