Those are a type of planaria. Usually excess food in the substrate will cause a reproduction explosion and they will become apparent to the eye.
They are pure protein and your fish should love them. They are harmless to your fish but most people don't like to see them. To make them go away do a deep gravel vacuum and a large
water change. You may have to do a couple of these a few days apart.
How often do you gravel vac? What is your water change schedule? what fish do you have in the tank?
Carol