Looks like a little paper loss where the crease is took off a nugget of color and the guy went in with a pen or marker and touched it up. Doesn't look that great of a job to me and I'm sure under a loop the paper loss would show up even more defined.
I agree, though. Don't sweat it. Nice looking card, even without the plastic holder.
If it bothers you, though, don't hesitate to send it my way.