He wonders what it means, when Allen says it wouldn't be wise to like him anyway. But when he compares it to himself, the secrets he doesn't like sharing, but he's obligated to provide when there's a chance he could really be with someone, he can't help but go pale.

Maybe Allen already knows what he is. Maybe Allen is being polite, like Kuro. And maybe he's wrong. But now, he has to ask.

"Are you... a hunter?"

Did he lose Kuro and yet somehow the hunters even managed to follow him here? To keep an eye on him?

He's trying not to be afraid of Allen until he hears his answer; but the possibility has him uneasy. He can worry about the rest of what Allen said after.