First, rinse and cook the quinoa according to the package directions. This should yield around 1 1/2 cups of cooked quinoa. You can do this step the day before you plan to serve this salad as you want the quinoa to be chilled before adding to the salad.
Juice one of the grapefruits. This should yield around 1/3 cup or maybe a little more juice. Combine in a jar with a lid with the olive oil, mustard, and a little salt and pepper. Replace the lid and shake well so that the dressing is well combined. Taste and add a little more salt and pepper if you think it needs it.
Segment the remaining grapefruit and chop into bite sized pieces. Remove the skin and seed from the avocado and chop into small, bite size pieces as well. In a large bowl, toss everything with the dressing just before you are ready to serve.