Relax. Your plant looks healthy. So there's a spot on it. Big deal. Yes the spot could be indicative of some sort of "problem". It could also be any number of other completely random (and harmless) anomalies. No-one's plant is perfect. Its kinda like your child has a freckle and you wanna take them to the emergency room lol. As I said your plant looks healthy. I dont think you should do anything to it to "fix"this spot. A lot of people try to fix things that aren't broke, and end up killing their plants.
You've already "flushed" your plant, and its not even old enough to fertilize yet for crying out loud! You're starting down a bad road. Please do not try to grow your plant. Let your plant grow itself.