Many commonly kept corals and inverts can cross from prized livestock to pests under the right conditions. For example, I will never again keep the BTA - just a large Majano to me. Xenia, clove polyps, green star polyps and mushrooms can all approach plague proportions. In my current system, green hairy mushrooms are my pennance. They grow everywhere and nothing eats them. I started out with one six years ago, and in a recent tank changeover eradicated over 200 of them .... and there were a 100 left. Deer, rats, cockroaches and green hairy mushrooms ....