This is what Eric told me when I 1st contacted him.
"Unless there are extenuating circumstances that I/you are unaware of, this is a "tissue-skeletal anomaly" that is more commonly known as neoplasia. They are common in Acropora - this type - and may multiply. To diagnose true neoplasia for a coral, you would have to do both tissue and skeletal sectioning and microscopic exam, but the gross signs are very consistent and also, like I said, a pretty common occurrence in the field and in tanks. That area may stay the same size or grow in size. Eventually, there may be some loss of tissue if the affected area expands, leaving a depression and dead area in the center of the lesion, but it is rarely if ever something that would cause the death of a colony.
Could you send me high resolution photos to add to my collection of such syndromes in aquarium corals?