Strange snail... maybe an Abalone?

[BlueAbyss]

So, back when I added the Calgary livestock to my tank, I found a tiny snail in one of the acclimation vessels. He didn't seem to be doing too well (snails need to be drip acclimated, I understand), but I tossed him in the tank anyway (his shell was probably 3/8" long). He has interesting iridescent colors in his shell, so I thought I'd give him a chance. Well, that was a month ago, and yesterday an hour or so before lights out, I see this greenish colored thing crawling across one of the rocks!

He's in the hollow in the middle of the pic...


Any ideas as to what he is? His shell doesn't look spiraled or whorled like a normal snail, and he's a little small to really identify him properly (I put him back in and probably won't see him again for a while). He's also at least doubled in size in a month.