Bring a large pot of salted water to a boil. Cook the elbow macaroni about 2 minutes past the package's al dente time, until noticeably soft.
Drain the macaroni and let it sit in the colander for 5 minutes, tossing once, until warm but no longer steaming.
In a small bowl, whisk together the mayonnaise, milk, rice vinegar, sugar, salt, and pepper until smooth and glossy. Reserve about ⅓ of the dressing.
Pour the remaining dressing over the warm macaroni and toss to coat evenly.
Fold in the grated carrot and squeezed-dry grated onion until evenly distributed.
Cover tightly and refrigerate for at least 8 hours, or overnight.
Before serving, stir in the reserved dressing a spoonful at a time until creamy. Taste and add a pinch more salt if needed. Top with sliced green onion.