I.D. this for me please!!

[7duster3]

So i woke up this morning just as my lights turned on and i saw this guy crawling on a new piece of rock i bought. He was moving and i didn't have enough time to get my camera so this pic is off of my phone. It was about the size of a penny maybe, completely flat and was moving, it was earthy tones with what looked like a darker head. I noticed him because he was over all the coraline. Also how normal is it to have coral and clams as hitchhickers on live rock?
What is this please and is it harmful?? thanks!

 

[cranberry]

Kindda looks like a chiton. Try to pluck him off and see if he looks like a snail underneath. If he doesn't come easily, or actually hunkers down harder, don't try to remove him. It indeed a chiton if coupled with that behaviour.

 

[teresaq]

I am going to take a guess at Chiton