Cook the macaroni in salted water until al dente. Drain and rinse under cold water to quickly cool the noodles.
In a large bowl, add the cooled macaroni, diced celery, red bell pepper, red onion, and chopped hard-boiled eggs.
In a small bowl, whisk together the mayonnaise, Greek yogurt, mustard, white vinegar, granulated sugar, salt, black pepper, and garlic powder until smooth. Taste and adjust the vinegar and sugar to your liking.
Pour the dressing over the pasta mixture and gently toss until all ingredients are well-coated.
Cover the bowl and refrigerate for at least 1 hour to let the flavors meld before serving. Adjust seasoning as needed and garnish with green onions.