That simply looks like a spot that was sitting against your brining vessel/bag and didn't get much circulation, or sticking above the surface of the brine, and the surface didn't oxidize. Even so, you should be fine! The salt & cure will continue to equalize even while it's out of the brine (even if it was a dry-rub cure, it still created a brine).
The purpose of overhauling (flipping) bacon, hems, etc. once every couple days is to minimize this.
Even with overhauling though, I get spots like that on occasion. As long as it doesn't smell spoiled, don't worry about it and smoke on!