Place the chicken in a large slow cooker. Add the beans, corn, salsa, chili powder, garlic, and salt. Mix to combine. Cook on low for 6-8 hours or on high for about 4 hours, until the chicken shreds easily with a fork. Combine the rice and water in a medium pot. Bring to a boil over high heat. Reduce heat to low, cover, and cook until the water has been absorbed and the rice is cooked, 35-45 minutes. Fluff the rice with a fork. Serve the chicken mixture over the rice. Garnish with parsley and scallions, if using.