How will I go to Guzhen from Haizhu Square in Guangzhou?

I think you may mean the Guzhen in Zhongshan, right?
If so, from GZ Fangcun Bus Station, there are frequent buses for Guzhen. To reach Fangcun Bus Station, you can take the local subway line 1 from Haizhu Square to Gongyuanqian Station and change to line 1 to reach Kengkou Station. Get out of the station from Exit B, you can see the Fangcun Bus Station.