Thanks lukying, Dylam, Dave and Cherry
Yes, the YSO bird must be a juvenile Fork-tailed Sunbird. It looks very similar to the one photographed by Cherry.
I think Olive-backed Sunbird is one to look out for in Hong Kong. It occurs in Hainan and along the coast of Guangxi and southwest Guangdong as far as Xitou (less than 300 km W of HK), where Jonathan Martinez has seen up to four on several dates in late autumn and winter.