Combine sugar and 1 tablespoon water in a saucepan; bring to a boil. Remove from heat; stir in nuts. Spread nut mixture onto a baking sheet coated with cooking spray. Bake at 350° for 10 minutes. Immediately scrape onto a sheet of foil coated with cooking spray. Spread evenly; cool completely. Lightly chop; set aside.
Combine 2 tablespoons water, 1/2 cup mango, juice, ginger, and 1/8 teaspoon salt in a blender or food processor; process until smooth. Stir in poppy seeds; set dressing aside.
Sprinkle scallops with 1/4 teaspoon salt and pepper. Heat oil in a grill pan or medium nonstick skillet over medium-high heat. Add scallops; sauté 2 minutes on each side. Remove from pan.
Arrange 2 cups spinach on each of 4 plates. Top each serving with 3 ounces scallops, 1/4 cup mango, and 1/4 cup onion. Drizzle each serving with 3 tablespoons dressing; sprinkle with 1 tablespoon nut mixture.