Question
I have patches of green stuff growing on my tree trunks.  At times, it looks like it growing on the sap that is dripping down the trunk.  It is definitely not moss.  It's more like a thin coloring over the bark.  Is this a sign of disease?  It's winter, so I can't tell if the leaves look sick, and I'm not sure what kind of tree it is.  It is in a forest area of central New Jersey.

Answer
Sounds like lichens. These are common and are not a problem to the health of the tree. Lichens are combination of a fungi and an alga. They are no threat to the tree so there is nothing to do. Here is a web link that will give you more information on lichens;  http://helios.bto.ed.ac.uk/bto/microbes/lichen.htm