By the process of elimination, it can't be a montipora because it doesn't have any verrucae or ridges. It is not a Stylophora for obvious reasons. Now it could be either a Acropora or Pocillopora.
To me the coensenum is very smooth, and the corallites are immersed. But it does look like there is some branchlet formation, from the picture. So the axial corallites and radial corallites are present. I will lean on saying it is an Acropora, but a very rare one indeed. I will try to find you an ID.