Invader

[BloopFish]

They are not the worst pest ever, but the biggest issue that I know of is that they spew their mucus web, and if you have enough of them it can cover and possibly smother your corals.

Exactly, when you have too much they smother your corals and prevent proper circulation around the coral. They also steal any foods you give them if you feed stuff like Reef Roids, then they'll multiply like crazy if they feed off of that.