The grey skin makes him stand out and makes Lottie curious. He has no Mask, that much is obvious, but he's clearly also not a regular person, either.

"Why are you grey?"

The question throws Louise, and for a moment she flounders, her face announcing that she very much wants to tell her child that you can't just ask people why they're grey, that's not DONE but then catches herself. People looking different isn't something that you just don't mention like she thought for most of her life. It's something most people cannot see. But this guy... So she'll just consider it fortunate that children get a lot more leniency when it comes to asking things bluntly, and wait for the man's reaction.