This is usually described as mucosal cell hyperplasia in corals, if the underlying skeletons formation also shows growth anomalies I have heard Borneman refer to it as calicoblasticepithelioma.
Not much solid information o
is out there regarding what causes it, some in the hobby believe its due to excessive UV light, but that is purely speculation.
In some cases have seen it come and go without and damage or distress done to the coral, but a few times though, it became the starting point for tissue recession and required "amputation" of the affected branch to save the colony
