Heat grill to medium. Season the salmon with ½ teaspoon salt and ¼ teaspoon pepper and grill until opaque throughout, 5 to 6 minutes per side.
Transfer to a plate and refrigerate for 5 minutes. Using a fork, flake the salmon into large pieces.
Meanwhile, cut away the peel and pith of the grapefruit. Working over a medium bowl, cut out the segments, adding them to the bowl as you go.
Add the greens, avocado, onion, oil, vinegar, and ¼ teaspoon each salt and pepper to the bowl and gently toss to combine.
Transfer the salad to plates and top with the salmon.