Hooboy, bad pic, and nothing to reference it's size. It could be anything. If I were to take a wild, I mean really wild, guess, I'd say it's some kind of soft sponge, maybe a leather. IMO, there's nothing wrong with leaving it in there for now to see what it is. Who knows, it might be something wonderful! That's the beauty and the risk of live rock, you never know what hitchhiker you'll get. I've grown sponges, ricordeas, mushrooms, leathers, even a rock anemone from out of nowhere. OTOH, there have been a few pests, but nothing that cannot be controlled once identified. I'd say leave it alone and see what develops!