This is probably a planaria worm or one of the other similar species.  Typically these are a result of poor cleaning of the gravel area resulting in polluted water.  Normally the gravel needs to be vaccumed with a gravel cleaner on a 1/3 per water change basis.  Once the critters are in there you have problems getting them out.  There are some chemicals available in aquarium stores for removing parasites and most worm species but depending on your fish (and snails or crustaceans such as shrimp or crayfish) it may also be toxic to other inhabitants of your tank.  To combat this problem it is suggested to vac as much as you can and change the water more often until you see the problem go away.  Also try to reduce feedings.  Planaria themselves are not really a bad thing if that is what you have and the presence is really just an indicator of too much debris/food particles in the tank.  If it turns out to be some of the other worms though this may be a problem though.  Scale back the feedings to give less food because it is likely building up in the water and the planaria are going after it and thriving.