What are the specific requirements for becoming a Mandarin teacher in Hong Kong?

It depends on where you want to teach!

If it is a private school, private tutoring or individual class, then it is simple, as long as you are a Hong Kong resident and you are willing!

If you want to teach Mandarin in a government-sponsored, subsidized or funded institution, you must be a Hong Kong resident, plus a minimum certificate of ability of the National Language Commission Level 3 and Level 6 certificates, and a recognized teaching qualification (teaching Law, teaching diploma...) and academic certificate (graduated from Hong Kong or a recognized university or college) can teach!