Place the frozen chicken in the crockpot.
In a small bowl, whisk together the teriyaki sauce, soy sauce, brown sugar, garlic, ginger, and red pepper flakes (if using).
Pour the sauce mixture over the chicken in the crockpot, making sure the chicken is well coated.
Cover and cook on low for 6-8 hours or on high for 3-4 hours, until the chicken is cooked through and easily shreds with a fork.
If you'd like to thicken the sauce, in a small bowl, whisk together the cornstarch and water. Pour the cornstarch mixture into the crockpot and stir to combine. Cook for an additional 15-20 minutes on high, or until the sauce has thickened.
Serve the crockpot teriyaki chicken over steamed rice, garnished with chopped green onions and sesame seeds, if desired.