Nikon d750 issues (cant move focus point and mirror is dirty?)

On the main button on the back which you use to move the focus point check that the switch on that dial isn't set to "L" which locks the focus point.
The dirt is on the focussing screen which you can see if you look up into the top of the camera through the lens mount. That is removable with a little sprung clip and can be cleaned with a lens brush or tissue. Use tweezers to remove it once it drops down in its little frame. Although you need to clean it with care it's not nearly as vulnerable as cleaning a sensor glass for example.
 
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