In a large mixing bowl, combine the cooked chicken, celery, and red grapes.
In a separate bowl, whisk together the mayonnaise, Greek yogurt, Dijon mustard, lemon juice, garlic powder, onion powder, salt, and pepper until smooth.
Pour the dressing over the chicken mixture and stir until everything is well coated.
If using, fold in the chopped pecans and green onions for added crunch and flavor.
Cover the bowl and refrigerate the chicken salad for at least 1 hour to allow the flavors to meld.
Serve chilled on a bed of lettuce, in a sandwich, or with crackers.