Dark Spots on Right Shoulder

HI, I'm a lurker here & have 3 beardies who have been healthy!! My older girl, Polly, who turned 8 on Oct. 28 has always been healthy, but around August I started noticing a patch on her right shoulder with black spots & a bit thicker skin than normal. The patch has remained the same size but the black spots are darker & a bit bigger. She has not had any change in environment or been introduced to anything new. She lives on tiles & has a Solar Glo light & has correct temp basking spot & cool areas to stay in. She hadn't been eating too much Oct-Dec & I attributed this to winter as she normally sleeps a lot & doesn't eat much.

I took her to the vet Monday who put her on baytril 2x day after a 5 min soak. I also started giving her a probiotic & putting betadine on the patch of skin. My vet wants $606 for a biopsy & I'm willing to spend the money if I have to.

I'm hoping its not yellow fungus (or any fungus) but the vet can't tell without tests. Could she develop this after 8 years in a clean environment? I'd appreciate any opinions you could give me as to the spots & if I should continue with her baytril treatment (worried its making her feel bad)

Here is the info from the vet. (finally figured out how to get it on here!)

Hi - did you ever figure out what the soot is? My dragon has one on his head