So I did some research on this today and actually found quite a few articles published over the last five years in regards to what causes white band disease (WBD) on corals in the Caribbean. WBD is when the coral starts losing skin and it spreads out from there. Long story short, it seems that each time it's studied, they find that a
pseudomonad population (a bacteria) shifts to an increasingly dominant
Vibrio carchariae population that attacks a weak spot of the coral tissue. They still don't know if this bacteria is the cause of the weakness or just opportunistic. The antibiotic ampicilin cures the coral of the bacterial infection the majority of the time.
They have also identified a single cell cilite (Philaster lucinda) that is typically found at the scene of the crime once the Vibrio starts attacking the skin. There's no evidence to suggest the Philaster lucinda is doing anything other than eating tissue after it has been attacked and expelled by the Vibrio. They have found that using metronidazole reduces the population of philaster lucinda below detectable levels, how ever does not stop the spreading of the disease. This why they believe that the Philaster lucinda is eating the already dead skin, rather than causing it to die.
I'll link a couple articles below.
So here's the TLDR: there is a type of bacteria in your tank that can become hostile to coral tissue in certain sps. They attack a weak spot on the skin and spread out. No one knows for sure if it causes the weak spot or just takes advantage of an injured coral. Also, there are little ameoba-like creatures that start eating the skin after it falls off. An antibiotic named ampicilin is the best treatment for the bacteria and metronidazole is used to kill the little ameobas.
Discusses how the philaster Lucinda eats after the Vibrio attacks the coral
Coral killer identified using experimental antibiotics | Royal Society
How to treat Philaster lucinda
https://www.researchgate.net/public...dangered_Caribbean_coral_Acropora_cervicornis