If I already in Hong Kong with USA passport, can I go into the China by using the 144 hours rules?

You can use the 144 hr free transit in mainland China as long as you will travel like country A-Chinese qualified city of 144 hr free transit-country B.

Hong Kong is considered a separate country from China, its treated no different than flying from the US to China. Land and sea boarder crossing would not be allowed. But you can for example fly Hong Kong to Beijing and then continue to another country other than Hong Kong. Just need to make sure the city you fly to in China is eligible for the 144 hour TWOV.