I agree on "Arctic" Warbler. The long supercilium and single wing bar fit this species, as does the date. It is difficult to get much detail though in these photos.
As Ho-fai mentions, Arctic Warbler has recently been split three ways (the split is now adopted by IOC, the list used in HK). The three "new" species are Arctic Warbler Phylloscopus borealis, Japanese Leaf Warbler Ph. xanthodryas and Kamchatka Leaf Warbler Ph. examinandus. Based on range, all three may be expected to occur in Hong Kong, but confirming the identity of individual birds will be very difficult, as the plumage differences are very slight. Song and call are likely to be the most important features in the field.
The "new" Arctic Warbler has been confirmed based on DNA of trapped birds in autumn, and calls heard in autumn may also fit best for this species. Songs/calls of birds recorded in the last two springs fit with Japanese Leaf Warbler.
