Snorkeler's Cube

Today I noticed a different form in the substrate, one I'd never seen before. The tank is 4 years old, so that was quite a surprise. I must have imported himwith the rocks and creatures I added on 9/Oct .

I recalled seeing something similar in shape in "What Invertebrates?: A Buyer's Guide for Marine Aquariums", which is a simple book but good enough for basic aquarists like me. The thing that looks like the picture is a Chiton, which the book says is safe. Looks like they eat algae film, including diatoms. That doesn't sound bad, does it ?

Now, I couldn't get a picture from the top of it, only from underneath, so I'm not 100% positive this is a Chiton.

If anyone has seen anything similar on their tank's glass, please post a comment. Specially if it wasn't a Chiton, but rather a pest!

Snorkeler