Answer: I would be surprised if you found a tree on your property without moss or lichen growing on the branches! My apple trees are loaded with it! The lichen doesn't feed off the tree, it's just clinging to the bark so it shouldn't cause you concern. There are ways that you can kill it, but unless you peel or scrape it off, the dead lichen will continue to cling to the branches until it weathers away. I'd just leave it alone. Eventually it will die off and slough off on its own. It really isn't harming your magnolia tree. The part you describe where two branches are emerging and leaving a groove is also nothing to worry about. As the branches continue to grow and expand they will fill in the empty spot and no more debris will collect there. I'd enjoy the tree for the beauty it brings and not worry about the lichen or areas between the branches. A very old, very large magnolia is a happy tree so I wouldn't do a thing to upset that!
Best wishes with your landscape!
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