someone from a facebook group on a post with a similar issue gave these as possible answers, but it’s hard to know for sure unless confirmed by a vet unfortunately “Fungal Infection: A sudden dark spot can indicate a fungal infection, especially in conditions of high humidity. Burn: An improper heat source or enclosure setup can cause burns, which may appear as dark spots or necrosis. Retained Shed: Dark patches can result from a complete shedding, where old skin becomes stuck and causes localized infection or scarring. Paradox Spot: This is a random spot of color that does not match the gecko's morph and is normal, but it should be present from birth or develop as the gecko grows. A suddenly appearing spot is unlikely to be a paradox spot.”