On these photos, the bird looks relatively small (but sometimes difficult to judge on a photo), the tail is long, there is an extension of white in front of the wing on the first photo, and broad white eye-ring. The overall 'jizz' of the bird is just right for Common Sandpiper. The barring on the wing coverts indicate that the bird is a juvenile - adults at this time of year should have plainer wing coverts.
Common Sandpiper is one of the commonest waders in Hong Kong, especially away from the intertidal areas of Deep Bay. It is worth watching these to become familiar with the characteristics of the species, so that you are more easily able to pick up the differences when faced with another species.
