Finely grate enough grapefruit rind to yield 1/2 teaspoon zest. Peel grapefruit with a paring knife, removing all pith. Working over a bowl, cut three-quarters of the grapefruit segments from the membranes, cut into bite-size pieces, and set aside. Squeeze juice of remaining grapefruit segments into bowl (you should have 1/3 cup juice).
Heat 1 teaspoon oil in a nonstick saute pan over medium-high heat. Pat scallops dry, sprinkle with 1/4 teaspoon salt, and season with pepper. Saute scallops until deep golden brown, about 3 minutes. Flip, and continue to cook until barely opaque all the way through, about 2 minutes more. Divide scallops between 2 plates, and loosely tent with foil. Reduce heat to low, and add grapefruit juice. Cook, stirring and scraping up browned bits, until reduced to 1 tablespoon, 2 to 3 minutes. Pour sauce over scallops.
Combine cucumber, lemon juice, remaining 2 teaspoons oil, the grapefruit zest and pieces, chives, tarragon, and remaining 1/4 teaspoon salt in a bowl. Shave fennel into bowl, season with pepper, and toss gently to combine. Pile salad atop scallops.
The crispness of cucumber and fennel is offset by tender scallops and chunks of juicy grapefruit.