An odd little snail

I just thought i'd comment about a little snail i've had since I setup my tank that was a hitchhiker on a piece of LR, he is a unique looking and extremely peaceful algae eater, also extremely timid and until recently rarely ventured far from his hole in the rock he came on and still only comes out at night, he only stays out for so long before returning to his home, the other thing that is odd about him as he carries a live spong on his shell that has grown with the snail as he has grown, this little guy has been a fascinating addition to my reef tank with his funny disposition, very timid and very easily scared, even sometimes seems like he does things to protect/benefit the sponge on his back, I'm afraid to take a picture of him with a flash as I don't want to cause him to return to hiding most of the time, if I can catch him durring one of his rare daylight ventures I'll add a picture of this funny little snail. It's little creatures like this that make this hobby all the more rewarding to me.