Preheat an outdoor grill for medium heat and lightly oil the grate.

Mix Sriracha, olive oil, garlic powder, and black pepper in a shallow bowl. Dip each halloumi cheese slice into Sriracha mixture and let rest 5 minutes.

Place each halloumi slice on the hot grill and cook for 3 minutes. Carefully flip slices and cook an additional 3 minutes. Serve immediately.