Tube WOrm......

[Rikko]

Tube worm/feather duster as you've already guessed. Without a photo it's impossible to make a positive ID.
I assume you didn't purchase this worm and nobody "surprised" you by buying it for you and putting it in the tank? If that was a hitchhiker or grew that big off your LR I'm quite impressed. If that's the case, your best bet w/o a photo is to look at tube worms commonly found in the country where your LR came from and see if you make any good matches.