kitty litter is the easiest to make a clay background from. the key is letting it soak for a day or two to soften it up. a clay background will not add moisture to a tank, but wll actually suck it out. clay dries very quickly and as it does, absorbs humidity. unless you are keeping the fbt tank at extremely high humidity levels (95-100%) it WILL dry out, no matter how many times you mist. that is why it is used in dart frog tanks primarily. Jason Grimm (whose tank you linked to) wrote some interesting articles on the clay background on dendroboard.

personally, i have built a few clay backgrounds and found they weren't worth it in a lower humidity enclosure. they dry out and crumble. with regular misting you can grow plants and moss on any surface. i have yet to find a background i couldn't grow plants and moss on, including concrete/grout