It only comes off if you put scotch tape on it then pull the tape off. In normal use nothing has come off. I've tried chipping the decal off with my fingernail and with a nail file and nothing came off that way.
Plus, the ingredients in the decal are body safe. I don't think there's a problem at all.
Mine came off right away with my finger nail...not sure why though. I did get mine when they first came out...and I simply could have had a defective one. I have several glass toys from Cal Exotics and love them. I did toss this one out, and wouldn't buy another...but that is just me. Plus I like the ones that come with a padded bag.