If your using R/O you got me then not sure why you would have a white crust.........Whats the source of R/O?.....Because I thought I was getting good R/O water from walmart at one time then I tested ppm and it was at a 100 and with the same meter tested some local store R/O and it was 6-10ppm.
For your spot question I think since you use RO water you should raise your Cal/Mag. For me I had those using RO in soil using a cal mag supp. But then I switched back to well tap and Im still using Cal/mag and necro spots are no more.
I bet those leaves that have the ongoing spots will be dead and limp in a couple weeks.