Acanthaster Planci - Crown of Thorns Starfish
A Starfish that rarely show up you can identify it by it's long spines that cover its body and up to 20 arms. The crown-of-thorns starfish has few predators and will eat corals by flipping them over. The spines are mildly venomous and may inflict a painful wound if the starfish is picked up with bare hands. They usually don't show up as hitchhikers but instead will show up for sale at LFS, so you need to be sure not to put one into your tank by mistake.