Cover capers with fresh water in a bowl and soak for 5 minutes to remove excess salt; drain.
Chop capers coarsely. Stir together capers, garlic, 2 tablespoons olive oil, lemon juice, and crushed coriander seeds in a small bowl. Season with ground black pepper.
Preheat an outdoor grill for high heat and lightly oil the grate.
Brush the halloumi slices with the remaining 1 tablespoon olive oil.
Cook halloumi slices directly on the grill grate until golden, about 2 minutes. Turn and cook on the other side until soft in the center and golden on the outside, about 2 more minutes.
Place halloumi slices on a serving plate. Drizzle with caper sauce and sprinkle with fresh cilantro leaves. Season to taste with salt and pepper.