Tonight while cleaning my crested gecko's food dish, I spotted a single very tiny light brown insect crawling around. I also noticed a dark dot on my crested gecko's underside near its armpit. The dot isn't moving, but it looks a lot like a description of a mites
Mites in that area would be moving or buried in the skin causing irritation, swelling, or lifting of scales. That doesn't look like mites to me. If you're really concerned you can replace the substrate with paper towels. It will be much easier to spot mites on white paper towels IF they're there.