Off heat, coat a grill rack with cooking spray. Preheat grill to high heat.
Place watermelon on heated grill; cook 10-20 seconds on each side until lightly charred.
In a medium bowl, toss arugula with oil; season with salt and pepper.
Place a watermelon slice on a small plate and sprinkle with 2 Tbsp cheese; top with another watermelon slice. Place 1/2 c arugula on top and drizzle with 2 tsp balsamic glaze; sprinkle with 2 tsp pumpkin seeds. Repeat with remaining ingredients to make 4 salads.