I agree that this is a
Plain Prinia, it looks like a juvenile, judging by the yellow flanges on the beak and the short tail.
Compared to a reed warbler, the structure is different (longer tail, shorter wings, blunter bill), the head pattern is less obvious and the upperparts are paler/sandier. You can also just see on this picture that the tail feathers have paler tips and paler sides to the tail.
Plain Prinia is a common breeding species in the reedbeds at Mai Po, and quite easy to see there. Manchurian Reed Warbler only occurs as a migrant in small numbers and is usually very hard to see.