In a slow cooker or crockpot, combine brown sugar, soy sauce, water, vinegar, sesame oil, salt, red pepper, cornstarch, garlic, pineapple, pineapple juice, and sesame seeds.
Whisk together until well combined.
Place meatballs in sauce, gently stirring to coat in sauce.
Cook on low for 4 hours up to 6 hours or high for 2 hours up to 4 hours, (see note).
Serve over rice with chopped scallions.
If you are going to be cooking the teriyaki meatballs for the longer cooking time, increase the water to 3/4 cup to ensure they don't dry out or burn. The number of meatballs you use will depend on their size. If they are smaller, you will have more than 16.