White spots on cucumber

Hi guys,

My cucumbers have some weird white spots on the bottom part leaves (not sure if it slowly spreading up).

Do you know what is it? And is that dangerous for plants?

In general cucumbers grow good and started to flower (there is no successfully pollinated fruits yet). Nutritient solution from Plagron (A+B+calmag+hydro roots), EC 2.5, pH ~6. Plants are in my winter garden, so in the night they might be a bit cold (+10 Celsius outside), humidity ~50%. DWC, air pump works 24/7.

Thanks for any tips!