Their poop can be pretty nasty, to be sure. Never pooped on his own back directly, but he has done things like walk through it, or fall off his basking platform into it. It’s absolutely possible that your dragon fell into it while climbing.

To be fair, if he poops in his enclosure, my dragon will run away from it and frantically claw at the glass (also called glass-surfing) until I clean it up. He HATES having poop in his enclosure!