A picture really is worth a thousand words. Your description will work this time though.
The first thing that a grower should do when they get clones from someone is put it in quarantine for a couple of weeks, away from ALL your other plants, so you can be sure you are not introducing problematic pests into your grow area.
Do they look similar to tiny white-ish catapillars with black heads? If so, then these are fungus gnat larvae. The beneficial nematodes should rid the medium of these pests... if there's enough beneficials in the medium. If this doesn't work, then try drying out the top 2" of you medium.The fungus gnats live and breed in the top .5"-1.5" inches of the medium. Drying out the top two inches of your growing medium will kill all pests in this area.. these pests need moisture to eat, live and breed... take that away from them and they die.
Also, keep the top of the medium free from any plant debris... this will attract fungus gnats and other destructive plant pests. To be sure if there are fungus gnat larvae living in the medium, simply put a few potato slices on top of the medium under the plant. The larvae will be attracted to the sugar and moisture of the slices and you'll be able to see them on the slice when you lift it off.
Any Pyrethroid insecticide added to the top of the medium will wipe out these pests as well.
Good luck to you!
