it does look like some sort of injury or infection...I had something like that happen to one of my females...but the tail was more dried out by the time I noticed. I saw it before but thought her tail was just dirty from the egg laying box..then I realized it wasn't dirt. I just had her drop her tail, this way the problem is dealt with quickly. To ensure no further spread of infection I dabbed some neosporin on her tail nub.
Of course doing this means your gecko will have no tail and the tail won't grow back. But having no tail has no health/breeding/looks impact on the gecko..or at least I don't care if a gecko has a tail or not. I find most crested gecko tails look like bird poop.
Make sure you cage is well cleaned, if any open injury on the tail, add a bit of neosporin, separate him/her from all other cage mates. If it gets worse see a vet or you can cause her to drop her tail.
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PHLdyPayne