Pat the chicken breasts dry and season both sides with the ground cumin, garlic powder, Kosher salt, and a pinch of black pepper.
Place the seasoned chicken in the slow cooker in a single layer.
Pour the cup of mild harissa sauce evenly over the chicken, cover, and cook on LOW for 4 hours (or on HIGH for 2 hours) until the chicken is very tender and easily shredded.
Remove the chicken to a cutting board or bowl and shred it using two forks.
Return the shredded chicken to the slow cooker and stir to coat with the cooking juices and remaining harissa; adjust seasoning if needed.
Serve warm with tzatziki on the side if desired.