What is this on my cucumber seedlings?
 

I recently torn my entire grow room Dow and sterilized everything because I thought it had powdery mildew. I just replanted the cucumbers and this white stuff has re-appeared, thoughts anyone please.

It looks like the leaves are getting wet with the emitter that close to the seedling. Wet leaves germinate fungal spores. Keep the foliage from getting wet both from splashing water as well as high humidity. Good air circulation will help moisture build up from humidity. Once the seedlings get bigger pull off the affected leaves. As they get bigger the foliage will get farther away from the splashing of the emitters.

Unfortunately I have ruled that out. This happened with my last crop of tomatoes and green beans even when they were tall and nowhere near the emitters. I tore the entire system down sterilized everything including the room with a mild bleach wash. I am beginning to think it may be from the nutrients? Nutrients seams to be the last thing that is the same from last grow to this grow.

What is the humidity level in the room, and what kind of air circulation is there.

Humidity
 

Currently very low humidity <50%, little circulation as the grow room is in the basement. Here is a couple more shots.

Another pic
 

Here is another of a cucumber seedling.

And another pic.

It's hard to tell from the pictures, can you wipe it off with your fingers, or is the discoloration inside the leaves.