I've experienced it once. It took out a large coral lps coral in about 2 days. Very healthy to skeleton. I threw out the coral and have never seen that stuff again. It happened so fast by the time I figured out what I was dealing with, it was too late. If I saw it again, I would QT the Coral immediately in an isolated tank. I hear that stuff can get out of control quickly. I think i was lucky it only took out one.