+1 They look like sea slugs with a "fingernail" shell on the back...opposed to chiton that have rows of plates going across the back and covering them completely.
I spotted a couple stomatellas in the back chamber of my cube, but they never lasted long in the display because of my wrasses. I noticed a few hitchhiked into my other tank on some liverock and they are multiplying like crazy. Great little cleaners...catch them cruising the glass every night.
After googling both and doing some research, I'm almost positive it's a stomatella. It has a thumb nail sized shell, that did not articulate or have sections. Thanks everyone for the help!!
I've had them in previous tanks. not the best looking snail, but I didn't mind them. Oddly enough, I don't normally see anyone selling them as part of a CUC group or individuall either.