I am citizen of China, with a mainland Chinese passport and US green card. I am flying directly from US to Hong Kong, without going to the mainland. What documents would I need? Is my passport enough?
Unless you are transiting Hong Kong for less than 7 days to/from Mainland China, your mainland PRC passport by itself is not enough to visit Hong Kong.
I believe you would apply for a Hong Kong entry permit from a Chinese consulate in the US.