Alex didn't reply to Rou.  Either he didn't hear him or he wasn't paying attention to him.  But before Rou could say or do anything else, he hears someone behind him say: "That wouldn't be the best way to talk to him."

It was the instructor Satsuki, who was also carrying a tray with something on it.  He comments to Rou while passing him: "Let me go talk to him.  He'll likely be back to your room soon."