They might be Planaria, or if you feed live foods, some are leftover and enjoying your substrate (Tubefix, Bloodworms? Do you feed anything like this?)
Also, worms can also be an indicator of poor upkeep. You need to vac the gravel to get rid of excess bioaccumulation, which provides the worms with plenty to eat and thrive upon.
So, do a good gravel vac regularly and the problem will be taken care of.