It means he rubbed up against something that irritated his delicate/tender skin. Think of it like a light bruise. My girl got a small one on her back today because she went under too low a branch and it grazed her side. She’s fine, but it just discolors their skin from the trauma. In my opinion it happens all the time when you let them free range because you cant control that environment as easily as their cage. At least not mine because they free range outside!
Heals pretty fast. May be only a hour-few hours to a day or two! Soon as the skin cells finally figure out no permanent damage was done! those darn cells! i would be very careful though and watch your cham closely the next few days though just Incase that fall caused a little more damage than you think. But his skin is just fine! The pattern leads me to believe he just scratched against a branch while exploring.