Frankly, it looks like there could be mites there. Can you take a pic on the undersides of the living leaves?
This could be also a result of overfertilizing. Even if you aren't fertilizing your plants it could be that the rainwater is getting old. Or the filter of R/O system isn't working right anymore. Some species are more sensitive to this than other.
please provide additional info: are you giving them daily night time drops to at least the 50s? how often are you fertilizing them? etc etc. as you see, the members are giving you way too many possibilities to give you a plausible/effective answer to your problem.
I collect my rainwater via the roof of my house. Who knows what it collects while running down to the rainwatertank. But nevertheless every serious orchid grower checks his water with an EC-meter. Most Orchids need more or less the same conditions like our neps. Using water without EC-meter is a blindflight.
In usually water the chids with pure distilled or RO. When feeding I use half distilled (or RO) and half well or rain water to avoid PH swings