Cook the rotini in well-salted water according to package directions until al dente. Drain and rinse under cold water. Let drain while preparing the dressing.

In a large mixing bowl, whisk together Greek yogurt, milk, ranch seasoning, and protein powder until smooth. Adjust consistency with more milk if needed.

Add the cooled pasta, tomatoes, cucumber, red onion, olives, and parsley to the dressing. Fold gently to coat.

Season with salt and pepper to taste. Cover and refrigerate for at least 30 minutes before serving. Stir before serving, and reserve dressing if making more than 4 hours ahead.