Not sure. I'm not an expert, but the polyps are small and the growth is branching, so it might be tubastrea faulkneri or even a branching dendro. D. arbuscula, D. ijimai or D. cylinndricaa. Tubastrea heads are usually smaller than dendros, but the picture does not have anything nearby that suggests size of the heads well. There are many, many species of sun coral and dendros, and not all of them are common in the trade...though an aquarium would have better access to the more rare species.
From my understanding, a firm id can only be made by looking at the skeleton. Whatever it is, it is beautiful!
