Line a baking sheet with parchment paper. Stir together crabmeat, bell peppers, mayonnaise, scallion, celery, coriander, cayenne, and arrowroot in a large bowl. Add egg whites and stir until well combined. Shape mixture into 10 3-inch cakes (about 1/3 cup each). Place crab cakes on baking sheet. Cover and chill for 30 minutes.
Heat 3 tablespoons oil in a large nonstick skillet over medium heat. Add half the crab cakes and cook until well browned, about 5 minutes. Flip and cook cakes until firm but tender, about 5 minutes more. Transfer cooked crab cakes to a plate lined with paper towels. Repeat with remaining cakes. Arrange 1 cup spinach-arugula mix on each of 5 plates; top each with 2 crab cakes.
Combine lemon juice and remaining 3 tablespoons oil in a small bowl. Drizzle over cakes and greens. Serve immediately.