Very difficult to tell from the picture but my guess would be Acropora prostrata if I had to pick between A. millepora. It is possible it is some other Acropora too.
A. millepora seems to have a more rounded branch tip while A. prostrata tips are more pointed like in this pic. I think it's a prostrata also. A. millepora also will grow into a table form while my prostrata's won't.