Its an atlantic vole urchin. Very rare invert from deep water trenches. Also iam full of shi@ lol you sure its not a snail? Possibly unlikely but a small octo?
First off, you should use r o water. Tap water is full of stuff you just don't want. Especially if you want coral. Once you add your live rock you need to test your water and watch for it to cycle. Only after that is it safe for anything.