What the hell is this stuff?! It’s everywhere!!! Even on the underneath the container lid!!! Zero light was getting in and lots of oxygen and cool water. HELP
That light cover is cardboard, should be plastic. The mist rising from the container meets the cardboard that is organic material --> something will grow. In this case, looks like mold.
Could be a chemical reaction from the metal. Looks like mold though. Treat the res with h2o2 and pray. Use hydroguard or something similar to protect your roots. Not much else can be done. Take out the entire system and scrub it if possible.