Add quinoa and a pinch of salt to a medium sized saucepan with 1-1/2 cups water. Place a lid on top then turn heat down to low and simmer until tender, about 15 minutes. Fluff with a fork then transfer quinoa to a large mixing bowl and cool slightly — can be made up to 3 days ahead of time.
In a large bowl combine cooked quinoa, crab, mango, avocado, and green onion then season with salt. Add ingredients for Lemon-Agave Vinaigrette into a jar with a tight-fitting lid then shake to combine or whisk together in a small bowl. Pour over quinoa mixture (may not need all of it) then toss to coat and serve.