I remember when it happened. Steve posted a thread on TRT that said something like "Caught a 6-FOOT worm last night."
I remember thinking, "Yeah, and I pulled a 2-foot sliver out of my finger the other day." I truly thought it was hyperbole - until I saw the pictures.
Granted, he had a massive tank, and the worm grew over a long period of time - but still...
I've seen a handful of Eunicids over the years, they are very few and far between, but they do manage to hitch a ride into a tank periodically.
I think the OP's worm is of the same, or very similar variety.
Jenn