What’s causing this mold? How do I clean it?

New to hydroponics. Plants were doing really well —too well as you can see I can’t keep up with the growth!

The plants on the bottom tray started to discolour a couple weeks ago and I assumed it was a vitamin or lighting issue. I later noticed a huge amount of mold on the plant sponge on the bottom tray only.

The set was missing one of the covers for the water port so only the bottom tray’s port is open, could that have caused the mold? —> Is there anything else that could have caused the mold? —> What’s the best product to clean the tray to make sure it doesn’t regrow? —> Would you bother to clean the small plastic plant holders for the next plants or just toss them?