We adopted two dragons that both had built up dirty shed on their tails. I gave them regular baths like every couple days. It took about two weeks to loosen up and finally come off. Keep pictures to see it you notice it changing. That will help you if you end up at the vet.
Sometimes if they sit in their poop too many times, the scales will get dirty and you have to wait for a shed.
I know nothing about fungus so I cannot comment on that.