To a 4- to 5-qt slow cooker, add the chicken and pour the teriyaki sauce over it. Season with pepper. Cover and cook on high 2½ hours or low 5 hours, until chicken is cooked through.
When chicken is done, halve or quarter the bok choy lengthwise and arrange in a microwave-safe baking dish, along with ¼ cup water. Season with salt. Cover with vented plastic and microwave 4–5 min., until cores are tender. Drain. Heat the rice according to package directions.
Using a slotted spoon or tongs, transfer chicken to a large bowl. Shred meat with 2 forks. Add cooking liquid from the slow cooker, stirring to combine. Serve over the rice and alongside bok choy. Garnish with the cilantro.