A white substance has developed on all but one of ...

I used to think it was salts as well, and it may be. However, someone else provided a better explanation: It's actually mold. Not the scary kind, but it's growing on the outside. Develops more with excess water. You can use iso to get it off, but I do this between grows instead of while the plant is still inside. Either way, it seems to make the bags breathe quite a bit less. Those pots don't seem to drain right.