i dont think thats yellow fungus --it looks like his coloring ---- but what I do see is future issues w/ the UVB--- health wise --- first here is some info on the UVB
Which
UVB light should I use?
The best
UVB light is a ReptiSun
10.0 fluorescent tube HO (Or Arcardia 12% in the UK).
Compact or coil bulbs do not emit the proper amount of UVB light for a bearded dragon, and should not be used. Other brands, such as the Zilla Desert Series and ReptiGlo have been known to cause eye problems and other health issues with bearded dragons, and should be avoided. The reason that some UVB lights (Zilla, ReptiGlo, coils/compacts, ect.) are considered dangerous is because the wavelength of UVB that they emit is shorter and more intense than the wavelength of the "safe" lights (ReptiSun, Arcadia). UVB lights should be replaced every 6 months. The T5’s are a strong bulb and only need to be replaced once a year – the T 8 every 6 months--- where as the T 8 are not as strong—they need to be placed inside the tank the T 5 10.0 bulb approx 10-11 inches from basking spot – the T 8 approx 8-10 inches from basking spot—the cover needs to be off the bulb for full effect of the UVB rays-- I would replace this ASAP--- next
please get rid of the sand --- are you in the states? Alot of people use textured non adhesive shelf liner from Home Depot Lowes etc......... its cheap and easy to clean vinegar/ water in a spray bottle mixed 50/50--- insects can get underneath----------- sand causes impaction -- the dragon needs to be able to sit directly under the UVB ---- so I would get rid of that coil you have and get the Reptisun above either the T 8 or a T 5 --- depending on what you get will determine the distance of the bulb in the tank -----
You can look up yellow fungus on line--google-- and look at the pictures they show --- but it looks to be his coloring to me--
Karrie