Lay the chicken breasts in the slow cooker. Mix the garlic powder, salt, pepper, and Italian seasoning mix together in a small bowl. Sprinkle the mixture evenly over the chicken breasts, patting lightly so it will stick to the chicken.
Sprinkle the Parmesan cheese evenly over the chicken, then pour the marinara sauce on top, covering the tops and sides of the chicken.
Cover and cook on low, checking after 3 ½ hours. (Or cook on high, checking after 1 ½ hrs.)
Uncover and sprinkle the mozzarella evenly over the chicken. Cover and cook on low for about 15 more minutes, until cheese is completely melted.
