E-liquid can become cloudy for several reasons. Often, they are cloudy when first mixed due to the mixing procedure introducing oxygen into the e-liquid. This cloudiness will clear up over a few days as the cloudiness forms into bubbles and floats to the top. Some e-liquids can stay cloudy. This is to do with a chemical reaction that can happen between certain flavourings, often apple or citrus flavours in particular. The cloudiness is nothing to worry about and will not affect the e-liquid in any way.