I am sure there is a textbook answer to this but I have always just trusted my own judgment for the most part.
Favia- or Closed brain, Lunar, Lunar brains, Moon Coral, etc etc (whatever you want to call it) it has smaller mouths with cleaner separation between lip and center. Also finer lines that run towards the centers. The skeletal structure shows through the coral more.
Micromussa- Has thicker flesh and more distinguished lines that run towards the center. With less skeletal showing through. The individual cells are more defined and have more detail.
Others will chime in and correct me if wrong, but that is what I go by.
I see Favia in your picture. JMO