Heat a slow cooker to Medium-High; stir the reserved liquid from the pineapple chunks, soy sauce, vinegar, and brown sugar together in the slow cooker. Whisk the cornstarch into the mixture until dissolved.
Mix the ground beef, egg, Worcestershire sauce, milk, bread crumbs, onion, salt, and pepper together in a bowl; form into 1-inch balls.
Heat a large skillet over medium heat; cook the meatballs in the hot skillet until browned on all sides, about 5 minutes. Place the meatballs into the slow cooker. Add the reserved pineapple chunks and green bell pepper.
Cover and cook until the meatballs are no longer pink in the center, about 1 hour.