Why does Li hate Mizuki?
As far as I know, there wasn't much of a revelation about why Syaoran acts that way in front of Miss Kaho Mizuki. Though for me, it would really make more sense if Syaoran would find himself attracted to her powers just like how he feels with Yukito. Nevertheless, Syaoran felt worried when Miss Mizuki is around, maybe, that's how Syaoran shows his alertness because of Mizuki-sensei's mysteriousness.