Looks like you are referring to the Coralline Algae on your rocks etc. I like it and its nice to have IMO, but you can get an Urchin if you dont want it around, over time it will munch it away.
Vermetid Snails. They're a natural part of a reef tank. You can crush their shells if you don't want them and the Cleaner Shrimp will munch on the remains. Most people just enjoy the diversity.