Appears to be a Red Rock Urchin Echinometra lucunter Depending on where you got your LR. Red Rocks are commonly found in the caribbean and Southeaster U.S. in warm tropical waters. They will stay on or near you LR at all times and feast on any plant material in your tank (mainly algae) including coralline. They will almost always have some red on their body (which you state yours does) juveniles (like in your pick) are usually almost all black with hints of red. Juvenile Red Rocks are commonly confused with juvenile Long Spined Black Urchins Diadema antillarum which is venomous. Juveniles of Diadema can also be a pupleish black but will lack any red hints. Color is hard to determine from your pic can you post another maybe? Have you touched the urchin already? If it was Diadema your hand would be in severe pain from its venom.
Unless it is infact the Diadema sp. Diadema spines are incrdibly sharp and will penetrate skin with ease releasing the toxin. BTW the toxin is not deadly it is just extremley uncomfortable, the contact area will feel like it is on fire and itch for a few hours to as long as a day. :x A quick way to ease the pain is to sprinkle lots of meat tenderizer or pee on the contact point.
MOST urchins can be moved safely and with ease as long as you know it is not harmful. 
