Its a Spionid worm. A hitch hiker. Its harmless when only a few. But several hundred can generally build a massive network of tubes in your sand bed using sand.
They are filter feeders. If you put a small pellet of food next to a sand tube, the worm will partially extend out, snag the food and attempt to pull it into the sand tube.
You can discover the network by simply turning over the sand by making a sandstorm using a pump. Otherwise, wait until the lights are out, and you may see them wondering (occasionally) along the glass looks for a new place to establish a new home.
Personally, they creap me out, but pose no harm to the majority of aquariums. I had a 100 gallon tank with a 5 year old sanded. When I did water changes, a strange phenomenon would occur. The hitchhiking starfish would release sperm/eggs into my water and cloud the entire tank, then millons of spionid tube worms would spring up, extend out, and sway in the current.
So now ya know. Good luck!!
