Grate half a cucumber into a bowl, then squeeze out excess moisture. In a mixing bowl, combine the grated cucumber with Greek yogurt, lemon juice, dill, garlic powder, and olive oil. Mix until smooth and creamy, then let it sit in the refrigerator for about 10 minutes to meld flavors.

While the tzatziki is chilling, shred cooked chicken. In a large salad bowl, combine the shredded chicken with chopped red onion, Kalamata olives, feta cheese, and chopped fresh parsley. Use a large spoon to gently toss ingredients together.

Once the tzatziki has set, pour it over the chicken mixture in the large bowl. Gently fold the tzatziki into the chicken salad until everything is well coated.

Taste the salad and season it with salt and pepper as needed, or add more lemon juice if desired. Let the salad chill in the fridge for 30 minutes before serving.