I have one more idea.
Might it be salt from your aquarium?
If you put pipette into the water then to the bottle again, you can contaminate it with water from your tank?
If not, you can assume that this is precipitation after all.
Still unusual, I took a picture of my old CB bottle.
Expiry date 9/2017, but I still use it, I switched to nano tank and was unable to finish it before expiration date.
It was kept in the fridge for some time, then in warm spot near my house boiler, and there is no precipitation.