Dear WW: Here is a URL showing picture of a crab disquised as a sponge. Down at very bottom, it gives genus name Stimdromia, but doesn't say clearly that that is name of the crab in the picture.
http://www.museum.vic.gov.au/crust/gbmovpet.html

Here is another, camouflaged with seaweed. Genus Naxia
http://www.museum.vic.gov.au/crust/gbmov743.html