This is - or so I have been reliably informed (thanks Mike and Raymond) the Green-headed Yellow Wagtail Motacilla tschutschensis ssp. taivana,
This is a subspecies of the (Eastern) Yellow wagtail.
The Grey wagtail, Motacilla cinereais, is actually yellow and much more colourful than its name suggests.
It has slate grey upper parts and distinctive lemon yellow under-tail. Its tail is noticeably longer than those of pied and yellow wagtails.
This one was shot at Sha Kok Mei, and you can often see them flying ahead of you in catchwaters, always out of camera reach!