James, the quickest way to remove a hermit crab is to put the crab and
shell into a half/half mix of (cheap) beer and water.  It won't be long
before the crab is totally limp and feeling no pain.  At that point it can
be, gently, removed from the shell intact without leaving anything behind.  
This is the only way I know to get a really clean shell.
As to the stuff on the outside, it has to be carefully removed - after
bleach treatment or whatever.