Cook the elbow macaroni according to package instructions. Drain and rinse under cold water to cool. Set aside.
In a medium pot, bring water to a boil. Add the shrimp and Old Bay seasoning. Cook for 2-3 minutes until the shrimp turn pink and opaque. Drain and let cool.
In a large bowl, combine mayonnaise, Dijon mustard, lemon juice, salt, and pepper. Mix well.
Add the cooled macaroni, shrimp, celery, red bell pepper, and red onion to the bowl. Gently fold all ingredients together until well combined.
Cover the salad and refrigerate for at least 1 hour to allow the flavors to meld.
Before serving, give the salad a good stir and garnish with fresh parsley.