I can see where one would guess Euphyllidae (formerly Caryophyllidaae)... and to some extent Trachyphyllidae (fan-shaped corallum resembles a free-living stony)...
but from pic alone (barely educated guess in such cases), I'm agreeing with Duce: this specimen has hallmarks of Dendrophyllidae and the like (solitary, aposymbiotic species... a bit reminiscent of Rhizos, Dendrophyllia, etc)
If you can give me an indication of where you saw them, we can track down the likely collectors and get a proper name for the import for ya.