Cook the Macaroni
Bring a large pot of salted water to a boil, then add the elbow macaroni. Cook according to package instructions until the pasta is very soft (a bit softer than al dente for authentic Hawaiian texture). Drain and rinse with cool water.
Create the Dressing
In a large mixing bowl, whisk together the mayonnaise, milk, apple cider vinegar, sugar, salt, and pepper. Adjust to taste if needed – the dressing should be tangy and slightly sweet.
Combine Ingredients
Add the cooked and cooled macaroni to the bowl with the dressing. Toss in the grated carrot, chopped celery, and onion, mixing well until all ingredients are evenly coated. If the macaroni absorbs too much dressing and seems dry, add a bit more milk or mayonnaise.
Chill the Salad
Cover the bowl and refrigerate the salad for at least 1-2 hours, or ideally overnight. This allows the flavors to meld and the pasta to absorb the creamy dressing.
Serve
Before serving, give the salad a good stir, and garnish with sliced green onions if desired. Taste and adjust seasoning with additional salt and pepper if necessary.