What is the white stuff on my clay pebbles? I thin...

That is salt build up, should not affect plants too much as they are pulling from the res below, I suggest that you cover the top of your pots with a non light penetrating material preferably reflective to prevent light leakage into your tote. I too use big box brand totes but the yellow tops shine light right through. I use a reflective insulation that also help keep water temps down.