"Ohh-- that's beautiful! He really knows how to draw well."

Things become much clearer once she hands him the paper and he sees the flower-- and naturally, the invitation with it. That causes Marui's eyebrows to lift for a moment. The roof of the south wing, huh? Marui knows what's waiting there, and he knows he's a dead man if he spoils it for her. It's more special than Flayn realizes (though if that giggle and eagerness to share her prize is any indication, she's elated enough all the same). Yukimura wouldn't invite her there if he didn't like her.

He also sees the color added to the rose, and he's not the expert on flower meanings that Yukimura is, but he knows enough to know that red roses mean love, pink ones mean admiration, and yellow ones mean friendship. That's certainly a different impression than what the circulating rumor gave. Sounds more like the Yukimura he knows, too. He's being enigmatic with her. Friendly in his own still-at-arms-length way.

He refolds it and hands it back to her with care, his smile full of warmth for just how happy she looks. "I'm happy for you. You're in for a treat."