What's the diameter of the polyps when closed? Acanthastrea maxima have corallites with 50 mm diameter, or about 2 inches closed. When fully open they can expand to 4 inches.
Your coral looks like the polyps are much smaller. Most likely a lord.
Because Maxima's are only found in Oman waters, and it's not a collection area, the chances of seeing one in captivity are pretty rare.
I think those pics were posted in this forum about a year ago, with the same discussion. You might be able to find that thread.