Can Anyone Id This Shelled Creature?

This is a bit of a strange one, not sure if the pics I took this morning will help or not. First set the tank up about 3 months or so ago, and a couple of days after the live rock went in, this appeared on the sand. It was a bright white little rock, I picked it up, nothing remarkable about it, I thought it was a bit odd but reckoned it must have fallen out of the rocks. Next day, it had disappeared, and looking around, I found it wedged into a crevice in the rocks, how it got in there I have no idea, and I couldn't see enough of it to get a picture.

And that was that, it hadn't moved until this morning, months later, when I found it out and on the side of a rock! Here it is, any ideas?

I'm no marine expert but thinking back to my days in the trade
one name springs to mind; dresser crab?