is there a sleeper bus from Hong Kong to Yangshuo? if so how long it take and what the schedule?

Oh, no direct long-distance bus can take you to the destination.
You are suggested to take a subway in HK, get to Lo Wu Station first. Then pass the custom and enter Shenzhen.
After that, take subway Luobao Line to Zhu Zi Lin Station, Exit A and arrive at Futian Bus Station. There are long-distance buses to YS. The departure times respectively are 16:30, 19:50 and 20:30. It takes 12 to 15 hours. And the ticket fare is CNY 190 to 230 per person.

Or you can take a direct flight to Guilin. It just takes around 1.5 hours. This is the easiest way for you to choose. :-)

Thanks, you know how long it take to get from Hong Kong to Shenzhen?

It depends on your exact location of HK.
And the time cannot be estimated because you also need to pass the custom.
Generally speaking, it takes 40 to 60 mins from Hong Kong to Shenzhen Luohu Station.