To use causes it should have an "and" as in:
Heavy fog hangs over Hong Kong due to a humid maritime airstream which is
affecting the south China coastal areas and causes visibility over parts of
Hong Kong to fall below 1,000 metres.
The reason why causing is used becomes clear(er) when eliminating certain parts of the sentence, without altering the meaning:
Heavy fog hangs over Hong Kong[...], causing visibility over parts of
Hong Kong to fall below 1,000 metres
It's just a matter of structure.