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.