Is it possible to go by ferry from Hongkong to Yangjíang? How long it will takes?

Yes, it is possible, but you need to transfer on the way. At HK China Ferry Terminal or HK-Macau Ferry Terminal to Zhuhai Jiuzhou Port first. There you can transfer to another ferry to YJ city.

HK to Zhuhai, it takes around 70min and the duration from Zhuhai to the destination is around 3h.