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These lichens don't cause issues, themselves. They can indicate that a tree has some sort of infection, and in some cases can be normal.

From this photo, I cannot tell which is the case.

If the rest of the tree is fine, and this is the only concern, then I wouldn't worry. If the tree seems to be lacking in health, then it will be necessary to check for pest or fungal/bacterial infections.

This article will explain how to get rid of it, if you choose to do so: https://www.gardeningknowhow.com/ornamental/fungus-lichen/treatment-tree-lichen.htm

This is the time of year for bacterial infections! Usually, this is the result of saturated soils that remain that way for quite some time.

You can attempt to use a fungicide, but it won't completely cure the issue, usually. This article will help: https://www.gardeningknowhow.com/garden-how-to/info/using-fungicides-in-garden.htm

Other than that, keeping all heavily infected growth cut, and keeping debris away from the area on the ground will help to kill off the bacteria.

A dry out will be helpful, if the weather cooperates.

Here are some articles that will help: https://www.gardeningknowhow.com/ornamental/trees/magnolia/magnolia-tree-diseases.htm

https://www.gardeningknowhow.com/ornamental/trees/magnolia/magnolia-tree-care.htm

It shouldn't affect grasses, unless there are multiple issues in the soil. Diseases for trees aren't usually the same diseases for grass, but this can indicate that there is an environmental factor that needs to be fixed before it will be suitable for planting.

Likely the area remains too wet without a dry period. If this is not the case, then I would make sure that this are is not anywhere near human feces, or septic lines.

If the soil is, in fact, infected with multiple diseases, then it will not likely resolve itself anytime soon. You will need to test the soil to know what it needs, or is infected with, though.

Here are a couple of articles to help you with this: https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/testing-soil.htm

https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/soil-ph-plants.htm

You may also need help from your local extension service. This link will help you to find the closest one to you:

https://www.gardeningknowhow.com/extension-search

This looks to be a mushroom! I see gills. Unfortunately, I cannot ID this one.

It could be that this mushrooms has caused the damage that you see, or it could be that it is taking advantage of the damage done by something else.

Either way, if you are noticing cracks this deep in the bark, it may mean that your tree is in decline.

You may attempt correction with a fungicide, but you may be watching your tree die, slowly, unfortunately.

Here is an article that will help:

https://www.gardeningknowhow.com/garden-how-to/info/using-fungicides-in-garden.htm

Sure! This article will help you to plant a Magnolia: https://www.gardeningknowhow.com/ornamental/trees/magnolia/magnolia-tree-care.htm

This collection of articles will offer much more information on Magnolia Trees: https://www.gardeningknowhow.com/category/ornamental/trees/magnolia