He doesn't actually turn around right away, he's so caught up in his own thoughts, but then he does and he turns to look at Minami with a completely neutral expression on his face. It's not exactly a great reception from his idol on any level, but Yuuri just can't manage... anything right now. Minami, unfortunately, probably doesn't understand that this is actually likely a better reception than he might have otherwise gotten, with Yuuri running away.

"Oh, I didn't know anyone else was here," and then, because Yuuri is still polite, "Congratulations." He can't sound excited about it, but he can at least not be a total asshole.