Garter Geek, It's a minor case of scale rot. They (kingsnakes in particular) seem to be pretty succeptible to it & it should be gone in a shed or two max. It can be something as easy to miss as a spilled water dish or a poop that escaped detection for a few days that it layed in proximity of, well you get the idea. Basically excess moisture that it can't (or won't) escape or some soiled damp areas it happens to be resting in.

It used to drive me nuts on my Kings as I'm pretty anal about cleanliness but it still happened once in a while. As long as you keep it clean & dry and as mentioned some neosporin or polysporin should also help it heal up quicker. Not to worry about unless it gets noticeably worse/larger.

Here's a link to some info and pics of a Cal King with a worse case than you're looking at that still wasn't bad either. It looks a bit different in this case because it's a wider area that's affected but you can see the discolouration and deformity of the scales:
T and J's Cornsnakes - Scale Rot