Favia or Acan?

[infamous]

I bought a tank second hand and with it came a rock partially covered in some coral growth. I can't figure out if it is a favia or an acan. I've tried to change it's positions in the tank (high current and low light, medium current and medium light, to low current and high light), but it doesn't seem to promote any more growth. I've had the tank for about 6 months now and expecting to see a bit of growth, but no matter what I do with the coral it doesn't seem overly happy. Does anybody have any suggestions about this? I've done some research on each type of the coral but I still can't figure out what it is. The tank didn't have any other types of hard corals in it before I bought it. Any comments or further questions are appreciated.