I'd agree with the other about it being a leather coral. I hope it opens up for you soon, then get another picture and post an update here. If it's attached to some rubble already, you can try gluing that rubble down to your scape or to a bigger frag plug.
I'd highly recommend you ask the LFS next time you go, exactly what it is.
I'm the same way btw, when I'm picking stuff up from the LFS. I get excited, throw my money at them, and skip my happy rear-end right out the door.
One of the things I do is take a quick video showing the piece I'm getting and then making sure I capture the sign if there is one, or have the LFS person say it out loud as I'm getting the video so I have a record. Then I jot it down in my notebook at home. I also recently started a digital " coral notebook", where I insert a picture of the piece and I don't leave the store until I've put in exactly what it is.
I think it's important to remember the species because of coral warfare. I didn't realize how crazy it can get, even with types that are supposed to be chill. But it's important to know what you've got so you know where to place it or where to place future pieces so they don't kill each other. Good luck!
