Last school year I worked at three different schools as an ESL specialist. 

At each school, there was at least one kid who would play around with me by calling me "Mr. Crab."  Each time I heard it I was a bit confused.  Why are kids who go to different schools and don't know each other coming up with the same nickname for me?

Well, I figured it out today.  There is a character on "SpongeBob Squarepants" that is named Mr. Krabs.  He is the owner of the restaurant where SpongeBob works.  Mystery solved.

I suppose that since I am the only person with "Mister" in his name that the kids know (usually their teachers are women,) that hearing or saying the word: "Mister" makes the kids think: "Mr. Krabs."

The good news is that they're not calling me "Crab" because of my personality.  I am definitely not crabby with the kids at work.