In a large bowl, whisk together the Greek yogurt, Dijon mustard, lemon juice, honey, garlic powder, salt, and pepper until smooth.
Add the shredded chicken, halved grapes, toasted almonds, and chopped celery to the bowl.
Gently fold everything together until the chicken and mix-ins are evenly coated with the dressing.
Taste and adjust seasoning—add more salt, pepper, or lemon juice if needed.
Cover and refrigerate for at least 30 minutes to let the flavors meld.
Just before serving, stir in the fresh parsley if using.
Serve on a bed of greens, in a whole-wheat wrap, or with crackers.