It look like some kind of chiton and it should be just fine in your reef.
http://www.wetwebmedia.com/mollusca.htm
"... Chitons however you might find stuck to shells and live rock. They are bilaterally symmetrical, ovoid shaped and flattened top to bottom; and as their name infers bear eight articulating plates dorsally. These innocuous creatures spend their days hiding from the light, adhering to a solid surface. They are algal scrapers who will do no harm to anything"