Can you post a pic?...leaves branches? cold it be lichen?
Can you post a photo of this. It will help us with your question,
Here is a link to lichens. It's a fungus, regardless and the same advice remains. If you live in a humid climate or the tree is now shaded, then it will form fungi of some sort, but normally harmless. Here is the link: http://wiki.answers.com/Q/How_do_you_kill_lichens_on_trees
The tree was cut back on neighbors property. I know longer have any green behind my patio. It is gone.