One negative fecal analysis does not mean 100% parasite free for life. The fecal sample submitted simply might not have had any eggs (false negative), or the animal may have recently enough been treated with the first/second dose of a *course* of dewormer (meaning it killed the eggs of the parasite while the adults or immature forms are still in the animal); or the parasite eggs may still be in the environment for reinfestation; or the parasite may be in a different stage of life cycle where eggs simply do not show up in a fecal float yet (seasonal, with or without having previously been dewormed). With any possibly WC animal--esp Mellers--you would be advised to check fecal analyses frequently, for life of the animal. I do with all my WCs, and only slightly less frequently with the CBs, once LTC WCs have entered the colony. Parasites are very pesky and persistent critters at surviving in their host and its environment. It is far easier to treat early stages of parasite infestation in a healthy cham rather than dealing with a deteriorating animal that is loaded with life-sucking parasites.
Hope this helps, and certainly not meant to be discouraging, though! You are really taking a great start to your chameleon's well-being by such proactive measures in observing his health. Best of luck to you, a Mellers is truly a beautiful species!
~Kerry