Heat gas grill to medium-high or prepare charcoal grill with medium-hot coals. Grill bread about 2 to 3 minutes per side or until lightly toasted. Remove from grill. Rub one side of toasts with garlic; brush with olive oil.
Reduce grill temperature to medium-low. Top each slice of toast with 1/2 tablespoon mozzarella, 1 tablespoon arugula and place another 1/2 tablespoon mozzarella over top.
Place toast slices back on grill, close cover and heat for 6 to 7 minutes or until cheese has melted. Top each slice with 1 teaspoon pesto; serve immediately.