There are many different species of fungi that can live on or in the barks of trees. They are usually not visible when the bark is dry but become so when there is lots of moisture that remains for a long time on the surfaces. The color of these fungi varies according to the species and can go from white to green or grey etc. In most cases these fungi are just saprophytes in the sense that they feed only on the dead organic matter of the barks.
There is only one case I know off where a white fungus has been observed on the bark of white ash trees that can cause serious damage to the trees. Here is a picture at this kink :
http://www.eco-systems.org/white_fungus_killing_ash.htm
The best way to find out if it's a fungus that is growing on your trees, and if it is pathogenic, is to have it checked in a plant pathology laboratory at your local agricultural services.