Preheat an indoor or outdoor grill to medium-high. Brush the chicken with 1 Tbsp. of the oil; season with salt and pepper. Grill the chicken for 3-4 minutes per side, or until cooked through. Set on a cutting board and cool a few minutes.
Place the mustard, citrus juice and honey in a small bowl. Whisk in the remaining 2 Tbsp. olive oil; season with salt and pepper.
Divide the salad greens among 4 plates. Cut the chicken breasts into slices and set on top of the salad greens. Top the salad with the mango, grapes and pecans. Drizzle the salad with the dressing and serve.