I think it's a male Daurian Redstart.
The white in the wing is not easily visible because this part of the wing is covered by the flank feathers, but I think I can just see some of the white on the base of the secondaries. I also think that the rear crown/nape is too pale grey for Black Redstart, particularly noticeable from the strong contrast between the grey nape and the black ear coverts.
Of course Daurian Redstart is a regular visitor and Black Redstart is extremely rare in Hong Kong, so Daurian is more likely to be seen here.
