That looks like an infection to me. First, use a wet Q-tip to gently wipe off whatever can be wiped off. If there is discoloration beneath that then it's an infection. Probably still somewhat mild. Mix 1 part betadine or chlorhexidine-brand name Hibiclens (betadine has more infection-fighting power in my experience) to four parts water and swab the area twice daily. If after a few days it doesn't look like it's improving, time to think about a vet visit. You don't want to waste time and end up with a severe mouth infection.
EDIT: Also, I'd put him on newspaper or paper towels for now. You don't want dust or debris from substrate sticking to the area and potentially irritating it more.
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