Cook the rotini pasta according to package instructions until al dente. Drain and rinse under cold water to cool.
In a large mixing bowl, combine the cooled pasta, crab meat, cherry tomatoes, red bell pepper, celery, and green onions.
In a separate small bowl, whisk together the mayonnaise, Dijon mustard, Cajun seasoning, lemon juice, salt, and pepper until smooth.
Pour the dressing over the pasta mixture and gently toss to combine, ensuring everything is evenly coated.
Cover the salad and refrigerate for at least 30 minutes to allow the flavors to meld.
Before serving, give the salad a gentle stir and garnish with fresh parsley if desired.