I'll be honest with you Boker. I've never seen a coral like yours before. To me, that's truly a one of a kind piece.(Until someone else shows one)
But I would not call it Acanthastrea Maxima. I do think it's more likely to be an undescribed species. Obviously you can't be 100% of a corals Genus just from a picture, but your piece has an Acanthastrea look to it. More than most of the unusual Mussides I've seen.
Any dead polyps on the underside? Try and get a photo of it at night, after the lights have been out for a few hours.