Heat a large straight-sided skillet over medium-high heat. Sprinkle scallops with 1 teaspoon salt and 1/8 teaspoon pepper. Heat 2 tablespoons oil in skillet. Arrange scallops in skillet, and cook on 1 side until golden brown, about 7 minutes. Flip, and cook until scallops are opaque, 30 to 45 seconds. Transfer to a plate lined with paper towels.
Reduce heat to medium, and add remaining tablespoon oil to skillet. Add garlic, and cook for 15 seconds. Add spinach, arugula, red-pepper flakes, and remaining 1/2 teaspoon salt and 1/8 teaspoon pepper. Cook, tossing greens often, until just wilted, about 2 minutes. Transfer to a platter, top with scallops, and serve immediately.