Stir cooked macaroni and cucumber in a salad bowl.
Whisk sugar, water, vinegar, mustard, parsley flakes, salt, and garlic salt in a separate bowl until sugar and salt have dissolved.
Pour dressing over the macaroni mixture and toss lightly to coat with dressing.
Toss all ingredients together in a large bowl, then season to taste with salt and pepper. Let chill for 2 hours before serving.