In a large mixing bowl, combine the shredded chicken, Greek yogurt, and mayonnaise. Mix until well blended.
Add the diced celery, chopped red onion, halved grapes, and sliced almonds to the bowl. Stir gently to combine all ingredients.
In a small bowl, whisk together the Dijon mustard, lemon juice, garlic powder, salt, and pepper. Pour this dressing over the chicken mixture and stir until everything is evenly coated.
Taste and adjust seasoning if necessary. If you prefer a creamier salad, you can add more Greek yogurt or mayonnaise.
Cover the bowl and refrigerate for at least 30 minutes to allow the flavors to meld.
Serve the chicken salad on a bed of lettuce leaves or as a filling for sandwiches or wraps.