Oddly enough, I just discovered some kind of clear gel like substance with black dots growing in the bottom of my bottle. I normally use one part Gatorade to two parts filtered tap water but lately I've been getting a refill from a public fountain in the park

So today I tried soaking it in hot water with dish water liquid and vinegar then scrubbing with a baby bottle brush and finally air dry. Let's hope that's the last I'll see of whatever that was.