Several weeks ago I added a new coral to my tank, a gregorian coral, and it came with a cute little white sea slug of some sort. I had pulled the slug off the coral and moved it to the other side of the tank. The tank is only 29 gallons, so that is not far, but it only took about a day for this little guy to find the coral again. He has never left it. He moves all around the coral everyday and stays on it. I cannot see that he's doing any damage to it. The polyps pull in where he's at, but those same polyps extend later when he moves. I had been watching for signs of damage. I am curious though.... what is this nudibranch and is he doing any harm to the coral? I've kind of taking a liking to him, so it's not likely I'm going to remove him.