I will tell you what we do with our dead coral we sell in our store to clean it. When it gets dusty and dirty sitting out on shelves we soak it for 24-48 hours in 100% chlorine. This whitens it back up nicely. Then we soak it in fresh water for another 24-48 hours to make sure all the chlorine was removed and it goes back on the shelves.
If I were doing it for a tank I would probably do about the same thing except I would dilute the chlorine to perhaps 50% or even less. Then I would soak it in FW
while using a dechlorinator and then soak it FW
again after that.