As far as I know, there are no cross-border buses from Maoming to Hong Kong. (I have checked the related info on the web. But no results) You may go to Shenzhen or Guangzhou by train first and then take the cross-border bus to go to Hong Kong. Guangzhou Yuexiu South Bus Station and Guangyuan Bus Station provides the buses to Hong Kong. Shenzhen Bao'an Bus Station and Huanggang Bus Station provide the buses to go to Hong Kong.

In Guangzhou:
Yuexiu South Bus Station
Location: at the crossing of Yuexiu Nan Lu and Dongyuan Heng Lu, Yuexiu District, Guangzhou
Bus routes: 7, 11, 12, 16, 35, 36, 54, 61, 65, 80, 101, 104, 125, 182, 184, 194, 264, 519, 541, Night 12, Night 16, Night 39

Yuexiu South Bus Station → Hong Kong
Departure Time: 08:00, 16:00
Ticket Fare: CNY80

In Shenzhen:
Bao’an Bus Station is located at 170 Baomin 1 Road, Bao’an District, Shenzhen. By Buses No.311, 315 and K651, you can get there.
Bao’an Bus Station - Airport Express Kowloon Station/Jordan/Hong Kong Airport/Wan Chai
The buses depart from Bao’an Bus Station every one hour. The service hours is from 08:00 to 20:00. Bao’an Bus Station is located Anmin 1 Road, Bao’an District, Shenzhen. The single trip needs about 70 minutes and the ticket fare is CNY60/70/160.