It could be 2 reasons:
1. chelated minerals, sometimes minerals dont mix well together and cause this to gel substance to happen, you will need to make sure there are no foul smells. I would change the DWC nutrients with less minerals maybe .25 ml/L.
2. rockwool rot, "Rotwool" from overwatering the rockwool, you will smell a noticeable foul smell. If you DO smell a stinky or rotten smell, its time to start over with new seed.
Most likely chelated, or gelled minerals if there is no stink smell.