Yes, a White Wagtail. This is the subspecies leucopsis, which is the commonest subspecies in Hong Kong. The other common subspecies, ocularis, differs by having a stripe through the eye and a paler grey back.
As well as subspecies differences, the appearance can vary according to time of year and sex of the bird. This is probably a first winter female - if you have previously seen breeding plumage leucopsis, especially males, they may look different (e.g. blacker back, more white in wing, more extensive bib).
