Cut, peel and removed the white pith from grapefruit. Discard them. In a large salad bowl, hold the grapefruit, one at a time and cut out segments between membranes. Add the shrimp, the sliced fennel and the cilantro leaves to the bowl. To make the sauce, combine in a small ball, the olive oil, the balsamic vinegar, the Dijon mustard, salt, and pepper. Drizzle over the salad and toss gently.