This is a juvenile Greater White-fronted Goose that has been in the Deep Bay area for a few weeks with two adults (probably its parents).
They look quite similar to Greylag Goose, but with orange legs, a slightly smaller bill, and browner plumage. The juveniles do not have the white foreheads of the adults.
Greater White-fronted is also a rare species in Hong Kong. You're lucky to get a good view for photos - when I have seen this family recent they have been very distant!
