Depends on what it is and how large it is. If it survived in the fish store tank, chances are it will live in yours as well. Have you seen any bristle worms? They look like flat red worms that have a long body, not short and oval or anything like that. They also have bristles protruding from their sides. If you have them in there, it'll be fine. I had a full size copperband butterfly flip out and get stuck in my rock which took me almost a week before I found it, and when I did, it was covered in bristle worms and almost completely eaten. I know for sure if they hadn't eaten it, it would have fouled my tank big time lol. I don't think salt water is as difficult as some people act, so I would just set back and let it take its course. Just make sure it sets for a while before doing much.