Is this a poop or regurg?

[Bogertophis]

I have a heat mat on a thermostat and two thermometers with probes in each of his hides, and one in the center measuring ambient. I'm almost certain temps are good.

You really need to be "certain", so you know if this is a "husbandry" issue or perhaps he is ill? Better (& cheaper) for you to narrow it down than the vet...