Who is "Fashion Police"?

not me.

"Fashion police" is a term which refers to the idea of an imaginary police force that make sure that people dress according to fashion. The term is made jokingly made on who criticise Fashion wear on clothes others wear. Fashion Police was the title of a television program hosted by Joan and Melissa Rivers on the E! cable television channel in the United States in which they criticized celebrities' clothing.

If someone says "the fashion police" are going to get you, it means they think your clothing style is horrible, that's all. it's slang and it's rude.