There are several different ways to do flood tables, but in all of them you need to keep light away from the roots.
Those are sad roots with lots of algae on them.
What is your flood/drain timing?
For best results, and maximum aeration, you need to flood for the shortest time you can, then allow them to stay drained for as long as possible - BUT - never ever ever allow them to actually get dry. Damp is perfect for roots.
Most people flood way too long. As soon as the roots get wet, its time to drain. Any time over that is just depriving the roots of 02. How long you can leave them drained depends on how your table is set up. If the roots are all in/covered by hydroton, you can go a long time as the hydroton holds a lot of water. On a plain flood table with roots only, it will be much shorter - maybe just a few minutes depending on how well its covered and sealed up, temps, humidity etc.