I need help diagnosing this issue

What is this? Is it a product of high humidity? The humidity rose to mid-50's and the heat topped at 30.2 Celsius. but only topped. The light is MH 500 watts at 24" away from plant. I just watered two days ago with a quarter strength solution. This is the 5th week in veg state. The species is white widow.

I get that from time to time during late veg and I've never figured it out.