Hi, 

Can you send a photo? Unfortunately, we can't determine what it could be just from the description. A white coating on bark could be several different things. It could be something like wood decay fungus which may mean that the wood is rotting, or something that isn't harmful at all like lichen. 

You can attach a photo to this response. If you can take a clear photo up close of the trunk, and one of the whole tree that would be helpful. Make sure the file size is 1 MB so we can zoom in clearly. 

If you have questions about attaching pictures, please let us know. 

Emily