I'm no expert but to me it looks like it could be a small crayfish. Crayfish are very cheap and I often see them being sold as feeders. It probably got tossed into the wrong "feeder bin" somewhere along the lines before it made it to the LFS. Heck, it could possibly even entered the ghost shrimp tank on it's own if there were holes in the dividers.
Can you get a better picture of it to confirm?