I don't think that's mildew.
Looks like a pH nutritional lock-out issue, or under-feeding deficiency. Sometimes dialing in the nutrition on young plants can be challenging.
That's good news if true. Would you recommend flushing the plants with pH-5.8 soft water? Willing to give anything a shot
But will definitely start lowering the EC-value. Better to underfeed than overfeed. But god damn I feel stupid if that's the issue