Mix dressing: In a small bowl or mason jar add sugar, mustard, vinegar, salt, olive oil and poppy seeds. Whisk to combine. If you're using a mason jar, secure lid tightly and shake. Refrigerate.
Assemble salad: Arrange greens in a serving bowl. Arrange grapefruit, avocado and shrimp on top. Drizzle over salad dressing. Serve immediately.