Brown on Toddler Lips

My almost 2 year old started waking up in the morning with brown stuff all over his lips about a month ago. At first I thought it was from the fuzzy brown sheet that he was using, so I switched that to a normal sheet. It stopped for a couple weeks, but this morning he woke up and it was there again. It comes off with some scrubbing, but I’m so confused as to what this could be. The only thing I could think of was his canine teeth working their way through, but during the day I never notice his gums bleeding.

Any ideas?