Like Leslea and the others said it is very hard to id. a frag of acropora unless you know where it came from and that person knows what the parent colony is. That said, this looks like a couple of species of acropora this frag may turn out to be. This all has to be taken into account that if the frag is wild it may not grow the same as the parnt colony in captivity. If it is a captive grown frag it may already have lost all parental characteristics to help you know what species this frag is.
So IMO, this looks to be either an acropora valida{not purple monster valida}, or acropora gomezi. I say these 2 species due to the up turned radial corallites and the big axial corallite. Whether this is close or not it's my best guess from cruising Corals Of The World, volumn 1 by JEN Veron.