Can't really tell for sure what that is but doesn't look like "scale rot" to me -if the color is reliable? I assume you've tried to wash it off gentle with damp cloth (?) in case
it's just dirt/substrate stuck on? Almost appears to be scabbed area (dried blood) so be gentle...is it possible she has got a minor cut there? I'd try dabbing with some
Betadine, in case there's any broken skin trying to heal- obviously she'd just rub away any sort of ointment (preferably Vetericyn on snakes), but she should be kept on
white paper towels for substrate if healing from issues, to reduce contamination of healing areas and so you can actually see if there's been any bleeding -it shows up
nicely on white paper towels.Obviously ask your vet about this too...you did say she's being treated for an infection? This is something your vet should be aware of...as is that swollen area if you've been trying to treat this on your own. That swelling is a bigger concern, quite honestly.