Congrats on the extraction! Thanks for posting this as i have seen similar holes in brains at my LFS now and then. BTW, nothing like a slimy green brain (pretty one too) on a fresh white rug.
nice job he should heal up great .i hope you dip it in some antibiotics so brown jelly doesnt accure.the crab would eat and keep the cut clean as he lived in it so it diddnt get infected and destroy his home so be carefull now that he is out of the whole so necrosis doesnt start.gl this should be stickied to help out future reefers with this prob.