It doesn't look like Yellow Fungus once it's already progressing, but it could be the start of a fungal infection, whether it be Yellow Fungus or some other strain. You need to watch it closely, and if it gets any larger and spreads at all, then you've probably got something that needs to have a Skin-Scraping test done by a reptile vet and be sent off to the lab for testing. Also, keep in mind that water/baths tend to cause fungal infections on them to spread, so if it persists or spreads, then you need to stop the baths completely until you get a Skin-Scraping done to confirm or rule-out an infection.