In several years of keeping and breeding chameleons (veiled; panthers; flapnecks; deremensis; quadricornis; jacksons; mellers; rudis; and pygmies), we have seen all sorts of skin conditions.
Several had to do with similar dark splotches. Some were simple bruising (most from a mating grip on the female, or slight injuries); others were conditions that required treatments.
Many of us have seen how easily a baby cham can get a blackened tail from a fellow neonate nipping on it, so it really doesn't take much to bruise the skin sometimes.
Of the dark splotches that can suddenly appear, we learned there are several skin fungi that can affect chams. We have seen this on panthers; jacksons and mellers. While a few spots developed a hard core in the center (like a small peppercprn) that had to be removed before treatment commenced, others remained smooth - but dk grey in color.
With our vet's supervision, we treated our chams with a 10% solution of providone iodine to cleanse the area and then apply a thin film of SSD cream. We found that most dark blotches would respond to the treatments by shrinking in size. It would take a while, sometimes several weeks, but most would go away.
There were a few spots that didn't respond, and these remained on the chams until their passing. We only had one cham necropsied that had splotches - and we could see that the discoloration went through to the inside of the chameleon. That particular cham passed from organ failure, and the spots remained unexplained.
If nothing else, cleansing the area with a cotton swab dipped in diluted Betadine would not hurt. Some minor irritations just need to be disinfected. Unfortunately, this would not be recommended for an area surrounding the eye - but under the chin shouldn't be a problem.
If you have a vet that you trust, it would be good to consult him/her about these mysterious skin maladies. We found that even jacksons can get too much humidity. Imagine that!
