Perhaps someone can ID that specific urchin for you.
Appears to be a Red Rock Urchin Echinometra lu****er
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.