It took only about a day for Chinese censors to crack down on posts from Americans who had flooded onto the Chinese social-media app Xiaohongshu. American TikTok users started downloading Xiaohongshu, which translates as Little Red Book and is known to Americans as RedNote, a little more than a week ago when it looked like TikTok might face a ban in the U.S. Their arrival was greeted by Chinese users and opened an unexpected window for free exchange. Early last week, officials from China’s top internet regulator told Xiaohongshu to keep a close watch on interactions between Chinese and foreign users and intervene if necessary, according to people familiar with the discussions.