These are symptoms of release of bacteria toxins and by products of dying tissue into the blood stream. (catsofaustralia.com)
When one particularly nasty specimen called turtle weed (Chlorodesmis fastigiata) rubs against A. millepora, for instance, wide bands of dying tissue girdle the coral. (sciencemag.org)
It was an opportunist and would sometimes infect ears and stalks when insects drilled into the plant, creating a convenient landing pad of dying tissue for the fungus. (scientificamerican.com)