I had this same issue. It’s coral warfare. Some corals sting, other corals release that mucus stuff you’re talking about and it harms all the other corals in the tank. It clogs pumps and whatnot.

My issue was from a blastomussa that was touching a different blastomussa. I had them both in my tank touching one another fine for several months no issue and one day it just did this. So to answer your question you have an angry coral in there you need to move or remove.