I really don't know anything about the hydroponics system , But
I believe you have leaf mold gray mold

, caused by a fungus,
From what I understand Grey mold as well as most molds are a result from a wet cool environment with not enough air circulation or ventilation primarily found in a nursery type environment ,
I would personally remove and destroy all affected leaves , I've read somewhere that baking soda in a spray bottle may help,
Hopefully more experienced members here will help you further, I'm sorry that is all I can help you with it.