Some corals are known to be pests based off their ability to take over a tank and cover every square inch of space. Things like Xenia, damicornis, mushrooms, kenya trees, and even BTAs. If a coral is happy it's going to spread no matter what species, some just do it faster then others so without manual removal they can overtake a tank fairly quickly. My favorite is probably pulsing xenia, it's fairly easy to remove (off glass and sand, rocks is another ballgame) and I'm always enraptured by their soft pulsing movement they add to a tank.
