That really does look like Thrips to me. The brown residue is their poop. Can you wash it off. Or is it adherent and hard to remove? Thrips are hard to see, you might need a magnifying glass, and with a pattern like the variegation on your juvenile TC, the damage might tend to blend in.
If the brownish stuff is adherent and won't come off, it could be a mold/fungus.But it really looks like thrips to me
