[Well, now it's too late to take it back. His smile widens and he reaches up to brush his bangs back, making no effort to hide his eyes.]

Please don't let it alarm you, my lord. I've been a grim reaper for some six hundred years, give or take a few decades. We don't all choose to reveal ourselves to humans, but this unnatural green in our eyes gives us away. Every time you've seen this color in someone's eyes, you'll know what they are, if or not they want you to see them.