Hello,
No worries, there is definitely no yellow fungus there, it is just his natural coloring. He is
very cute, too.
So you have a T5 Reptisun tube bulb? I would also try to get it mounted inside of a good
light fixture too, all of the way to the end directly beside your basking bulb. That is perfectly
fine to have it mounted inside of the tank, within a 10-12 inch distance from your dragon.
Placing it in a light fixture above his head will help to direct the UVB directly downward onto
him instead of allowing the UVB to refract out & away from your dragon.
I hope he is eating well & adjusting to his new life with you.
Tracie