Spray a nonstick skillet with cooking spray; heat over medium-high heat. Cook and stir garlic in hot skillet until lightly browned and fragrant, about 1 minute. Move garlic to one side of skillet.
Place swai fish into the skillet and season with Greek seasoning. Spoon garlic onto top of fish. Cook until edges of fish are white and bottom is browned, about 5 minutes. Flip fish and cook until fish flakes easily with a fork, about 5 minutes more.