Instead of calling them out, you could email them personally. I was "called out" by someone once who was sure I'd stolen their concept, but I'd never heard of the person or read the fic. I found it after, and aside from some very surface similiarities (e.g., AUs characters with similar occupations), there was absolutely nothing in common between the two. You can't really own AU tropes. Not enough info on the nickname to know if that's something equally likely to happen or something highly specific no one else could possibly have stumbled upon, but I've also noticed that nicknames seem to catch on in a fandom once someone's used them.

Point is - don't call them out in public until you know for sure they've copied your fic. Try some non-public ways to gather info first.