Little white worms?

they sound like planaria. They are most likely harmless, as the harmful kind (a.k.a. flukes) would be stuck to your fish somewhere. The free-living ones that you have are thriving and reproducing in response to too many excess nutrients in your tank. The solution is to reduce feedings and increase water changes/gravel vacuumings.

If you need more info, search the forums for "planaria". There are tons of threads on this.