BLUE TONGUE SKINKS

Hi! I just noticed this morning that my northern has some dark specks on the sides of his mouth, and I’m not sure what it is. The inside of his mouth looks healthy, and he hasn’t bled or had any loss of appetite. I use 2 kinds of substrate: aspen shavings and fir bark (baked at 200° for 3 hours to kill any mites), so I’m thinking it might possibly be bits of the fir bark from burrowing in it.
He’s also about to shed, if that makes any difference
What do you think it is? Should I take him to my vet?

Could be dirt, or minor scale abrasion.
Mouth rot is typically accompanied by swelling, and this doesn't appear to be mouth rot.

Dirt, food stain, abrasion.