I've always keep a fresh tub of bok choy or such in with my beardies so they can snack at will. I used to be very good & plant them into the topdressing sand so no dirt from the plant nursery was visible, but now I just pop the whole thing in straight from the nursery
I have checked with this nursery re insecticides etc. It will depend what substrate u use at the moment - if it's a sand of some discription then as said, just make sure the top layer is this - does that make sense? :?