Adult moorhen is blackish with white undertail coverts and a white stripe along the flanks. They also have a red bill with yellow tip.
The bird in this photo is a first-winter individual, which is plain brown above and whitish underneath - the white undertail and white flanks are also visible (as on adults). The brown plumage will moult into adult-type black plumage over the course of the winter.
Mai Po (especially the waterfowl collection) is a good place to get an idea of the variation in Moorhen plumage.
Watercock occurs in HK mostly as a migrant (mostly May/June and September/October). It is considerably larger than moorhen. Most birds seen here are in female/juvenile plumage, which is much more strongly marked than moorhen, with pale fringes to all feathers on the upperparts. Breeding plumage males are unusual but distinctive for being very large and all-black.
