It's difficult to tell from your description, but it might be a nudibranch (sea slug). However, it might also be a chiton. Chitons are flat, slow-moving critters which have a "shell" made up of slightly overlapping plates. This makes them sound like an armored vehicle, but they really are quite flat, and pretty similar to the description you gave. They're common hitchhikers on live rock and are believed to eat algal slimes.
Basically, nudibranchs will be completely soft, and quicker-moving, while chitons have a tougher, leathery or armored feel, and are pretty doggone slow.
I used to have a link for a nudi ID site, but I need to check and see if I still have it at home (I'm at work).
Kevin