Is this a commensal crab ?

[hillegom]

Here's a link I picked up, lots of pics, but yours isn't on the list.
http://www.flmnh.ufl.edu/reefs/guami...dae/index.html

here is a pic of a porcelain , scroll down. I don't think that is what you have
http://www.xtalworld.com/Aquarium/hitchfaq.htm