In a 3 1/2-quart slow cooker, combine frozen corn, roasted red pepper, and the 2 tablespoons green onion. In a small bowl, whisk milk into sauce mix. Add sauce mixture to cooker; stir to combine (frozen chunks of corn may remain).
Cover and cook on low-heat setting for 6 to 8 hours or on high-heat setting for 3 to 4 hours. Stir before serving. If desired, garnish each serving with additional green onion.