Arctic Warbler I think. It looks relatively large for a Phylloscopus, with elongated shape, long wings, short tail and relatively large bill. Wing bar fairly narrow (not as broad as Yellow-browed), vague streaking on the breast and long, thin supercilium. The bare part colours are a bit tricky to judge because of the light but it seems to have brown legs and mostly dark bill with pale base.
Among the rest of the Arctic complex, Kamchatka Leaf Warbler still hasn't been recorded in Hong Kong, and Japanese is very rare, with only a handful of records. Arctic Warbler is probably the commonest Phylloscopus in Hong Kong in mid-May - other species have already moved north, but Arctic is quite a late migrant.