My first instinct would be to clean the tank and start with new everything, after you disinfect it. Most likely those worms are up to no good and they will multiply quickly.

Make sure your reservoir has NO light coming in - that helps stuff grow in your water that you don't want. And make sure there is no other way for stuff to get in there as much as is possible.

are you sure they are worms, or could they be mosquito larva? I noticed that mosquito did reproduce in my hydroponic reservoir if I didn't have enough aeration/splashing. I actually had to add mosquito dunks to the tank.

Here is a link that might be useful: My Hydro Page

my thought was mosquito larvae too.
your system should be relatively well sealed to keep them out. I used to put window screen over the top of my bucket to keep them away.
I don't think it'll hurt the plants but ,full grown, they can leave some pretty nasty welts.

I found out that they are mosquito larvae. I have patch up all the hole now to keep those damn mosquito away.