Hopefully its not this but this a direct from the website
Burrowing Worms : While unable to obtain the actual organism that created the below problem, it is apparent due to the formation of calcified tubes that a worm species is most likely the cause. As this pest species first infests a coral colony, it first builds its own structure on top of the corallite's walls, which block light and water flow to the underlying polyp. The polyp then slowly dies over the course of weeks and months allowing these and other organisms to further colonize the coral skeleton substrate, effectively killing the coral colony little by little.
Formation of dead and soon to be dead areas on the coral colony. Even in seemingly healthy areas of the colony, these organisms are already present as individual formations on top of the corallites. In the most affected areas, the organism has burrowed through the walls of the corallites.