It's emulsified oil. If the water content is no more than condensation from successive cold starts after which the engine did not get hot enough to evaporate off the condensation caused by the cold start, then not a serious problem. The mayonnaise will probably disappear over the summer. But if the coolant level drops then you may have a cylinder head gasket problem or a crack in the cylinder head water jacket, or a problem with the water-cooling system of the turbo.