If it survived the molt it's probably not just dirt. Is the gecko acting normal otherwise—eating fine, moving around normally? Might be worth just monitoring it for any changes.
The trick is dusting way lighter than you'd think. Most geckos spit out heavily powdered insects because they taste chalky and bitter. Try putting just a tiny pinch of supplement on 3-4 bugs, shake for barely a second, and feed those. You'll probably see an immediate difference. If your gecko...