I have never seen this and I know absolutely nothing generally speaking, but my bag of worm castings has something in the fine print that states that some eggs and other worm stuffs might be mixed in with the castings. Perhaps this worm was live the whole time, or maybe he was an egg that hatched! I don't think an earthworm should be a problem though. If anything it will probably keep the dirt soft and full of nitrogen-rich castings.
I think every pot in my room has at least 5 to 20 worms in it....that changes every few days because they migrate to other pots when the lights go off after a good watering...
I buy red worms to add to the soil on my recycles if my worm bin isn't rollin'....
The pots with the most worms host the best plants IME....but this is an indoor environment we are talking about here.
Is there any reason not to add red worms directly to all the soil ..? I am so fucking lazy I could maybe maintain a half assed bin/resting batch of soil I would even put shredded paper on top for them...maybe