That large, its wild harvested. At best, maricultured, though that doesn't make them any easier to keep in aquariums.
If you do keep it, your best bet to keep it alive good is through frequent target feedings. Food like cyclopeeze, very small so it can eat it. Cut a plastic liter bottle in half, put the top half over the coral, and squirt the food into the bottle's top. That way the food is kept right over the coral allowing it to eat (it does so very very slowly) and the food isn't carried away by water currents or eaten by fish before the goni can eat it.