I get that same thing if i have plants near my pool. It only grows on the stems. If I move them away and just wipe off the white stuff it stops.
First year i saw it i kept the plants near the pool all season and it never went away when i was trying to fight it with green cure. Never spread past the stems.
https://www.growweedeasy.com/3-ways-to-avoid-cannabis-mold This link shows the same type of growth and says powdery mildew.