Often with reef tanks we buy rock that comes right out of the ocean and so it's full of all sorts of life that we call hitchhikers. Most often they are beneficial scavengers but occasionally someone will get a crab or mantis shrimp that will kill fish or eat corals so it's always good to check. I've had some coral eating snails and nudibranchs and a worm that ate my xenia coral but have lucked out that these have been the worst I've encountered and were easily removed.
One of my favorite is the peanut worm. I have one that has been in the same hole in the same rock for 9 years. It sticks out its snout and sort of vacuums up the rock and sand in the area around its den. They are black and white striped so look sort of fun. Inside the rock, much like the spaghetti worm, they are quite ugly but outside not so much. All the infauna of a reef tank is probably more fun for me than the fish and corals to be honest.
