Preheat a grill or grill pan over medium heat. Brush halloumi slices with 1 tablespoon olive oil.
Grill halloumi for 2-3 minutes on each side until golden brown grill marks appear. Set aside.
In a small bowl, whisk together honey, hot sauce, red pepper flakes, lime juice, and remaining olive oil. Season with salt and pepper.
In a large bowl, combine watermelon, mint, red onion, and pistachios. Gently toss.
Top with grilled halloumi and drizzle with the spicy honey-lime dressing just before serving.