Worms in the aquarium

I need some help identifying worms in my aquarium. My tank is heavily
planted, and I recently found some worms moving along the substrate. They
are reddish-brown, about 2 inches in length and 1/8" in diameter. I would
supply a photo but they disappear very quickly.

My questions a

1. What are they?
2. Where would they have come from?
3. Are they harmful to the fish?

Thanks in advance.