Looks to me like a Hainan Blue Flycatcher (they are paler underneath than Small Niltava and this bird has no "niltava spot" of shiny blue on the side of the neck)
There are a few winter records in Hong Kong, but its a pretty amazing record with all this cold weather to have a summer visitor staying!
Cheers
Mike K
Mike KilburnVice Chairman, HKBWSChairman, Conservation Committee