From what I can see in the picture, the hole looks to be an old one and well healed-over. If that is the case, you don't need to do anything - the tree has already taken care of itself. It looks like something may be using the space as a nest, which would not be uncommon. Sealing over tree wounds is no longer recommended, as it may trap fungus diseases in the wound.

Thanks so much for your response.  What can I do about the heavy moss cover, if anything?  

I wouldn't worry about the moss - it is doing no damage, and is indicative of a shady, damp location. Even if you remove it, with those conditions it will return.