Why does the thawed fetal bovine serum appear cloudy?

The serum producing process includes allowing the blood to clot, then removing the serum by centrifugation.  Fibrinogen is a large fibrous glycoprotein found in blood which functions in hemostasis and injury healing, and is essential to forming clots.  Fibrinogen is insoluble; the conversion to fibrin occurs during clotting.  Not all of the fibrinogen is converted during clotting and some may remain in the serum during filtration.  Thermodynamic changes will cause fibrinogen to convert to fibrin, such as incubation at 37°C or more for prolonged periods or the freeze/thaw processes.  As the fibrinogen is converted to fibrin during the thermodynamic changes, flocculent material or turbidity will be observed.

If you believe that your serum is contaminated, try plating it out or putting some in media and incubating it. DO NOT put a bottle of thawed serum directly into an incubator; always add it to cell culture media first.