Grey-plumaged Little Egrets like this turn up occasionally in Hong Kong. I think general opinion is that these are a rare darker colour morph of Little Egret, but it is not very well understood why some birds are this unusual colour.
These birds could, of course, be confused with Pacific Reef Egret, but note that this bird is paler grey with more extensive white, and that the structure is typical of Little Egret with a long, thin bill and relatively long legs.