To prepare vinaigrette, combine first 6 ingredients, stirring with a whisk.
Preheat oven to 400°.
To prepare salad, arrange the sweet potato in a single layer on a baking sheet lightly coated with cooking spray. Lightly coat potato with cooking spray; sprinkle with 1/2 teaspoon salt and 1/4 teaspoon black pepper. Bake at 400° for 25 minutes or until potato is tender, turning once after 15 minutes. Cool completely.
Sprinkle scallops with 1/4 teaspoon salt. Heat a large nonstick skillet coated with cooking spray over medium-high heat. Add scallops, and cook 2 minutes on each side or until done. Combine sweet potato, greens, grapes, and red bell pepper in a large bowl. Drizzle vinaigrette over potato mixture; toss gently to coat. Place 1 1/3 cups salad mixture on each of 6 plates; divide scallops evenly among salads. Sprinkle each serving with 2 teaspoons pecans.