[Don't think Yuki didn't immediately notice small differences. He's just not commenting until he has the full picture. The shock of being displaced is annoying, but real. At least this is his Momo, different or no. At least the compliment works out as it should, and Momo's not some kind of weird doppelganger of his partner or something.

The information is... Okay, while it's shocking, he's not doing anything but accepting it. There's no reason to doubt Momo's honesty about this, despite the fantastic nature of it all. And, looking around, Yuki's not stupid. He can see they're far from home, can sense the differences, a kind of shimmering strangeness in the air.]

I believe you. I'm here, but I just saw you a few minutes ago backstage looking completely different.

[No one at home will know they're gone.

Which means.]

How long have you been here?

[As if Yuki can avoid getting right to the heart of it.]