I was thinking that this was some kind of fungus or sponge. But it has already killed my montipora. It grows fast. Any help on what it is and how to stop it from killing my corals would be greatly appreciated.
As you described, it is similar to a substance that has affected some coral in the pacific known as cyanobacterial pathogen — a bacteria that grows through photosynthesis and a fungus that affects coral spreading at the rate of 1 inch a week on the coral it infects.
Are you able to blow this loose with a turkey baster? I ask because i believe this is chrysophytes. It is not common but does mat rock and coral in the way your picture shows. Are you Carbon dosing by chance?