dirty skin between scales?

[Zoe]

I was looking over my rescue boa more carefully (i've been keeping contact to a minimum - just applying polysporing to his nose and major cuts and such) and I noticed that his scales aren't as "tight" to his body as normal. Mostly along his sides and on his stomach. They don't stick straight out per se, but they just seem loose and easy to push backwards (not that I tested their resistance or anything). Anyway, it looks a little dirty between the scales. In particular, near his face, there is even a scale missing and its all brown there and between his other scales.
Any ideas what this could be? I'm giving him a bath in a betadine solution right now. Is there anything else I can do? At 10$/tube I can't really afford to coat him with polysporin :P but should I apply it to the worst areas like on his chin?

Thanks!
Zoe