Bring a large pot of heavily salted water to a boil. Add the macaroni and cook according to the package directions, adding 2 minutes. Drain and place into a large mixing bowl. Stir in the apple cider vinegar and allow to cool for 15 minutes.
Meanwhile, mix the mayonnaise, milk and granulated sugar in a medium bowl. Fold into the macaroni along with the carrots, scallion whites and light green parts, celery, onion, 1 teaspoon salt and a few grinds of pepper. Refrigerate until chilled, about 2 hours and up to overnight.
Before transferring to a serving bowl, give it a good stir and add more milk, 1 tablespoon at a time, if needed. Taste and adjust the seasoning as needed. Garnish with the scallion greens.