Place frozen meatballs in your slow cooker.
In a medium bowl, whisk together teriyaki sauce, reserved pineapple juice, brown sugar, rice vinegar, garlic, and ginger until well combined.
Pour the sauce over the meatballs and stir gently to coat.
Cover and cook on LOW for 3 hours, stirring once halfway through.
About 30 minutes before serving, add the pineapple chunks and red bell pepper to the slow cooker. Stir gently and continue cooking.
Switch to WARM setting for serving. Garnish with sliced green onions and sesame seeds before serving with toothpicks.