It is a Grey-tailed Tattler. Compared to other sandpipers in HK, these tend to look rather uniformly grey all over, including the rump and tail (there is some barring on the tail, but not as obvious as other species). Other good features are the relatively short, thick, yellowish legs, the heavy bill and the long wings.
Green Sandpiper should be much more contrasting - darker above and whiter below, with a well defined separation between these on the breast. The rump would be white and the tail white with obvious dark bars. There should also be an obvious white eye-ring. They are also mostly a bird of freshwater wetlands, and are very rare on the mudflat (and in fact are uncommon on the reserve at Mai Po, being easier to see in fishponds nearby).
