The image area you are pointing to is the fresnel focusing screen when you look into the eyepiece this is the surface you are looking at. One side is frosted and the other has the fresnel surface which bends the light to offer a better image onto the frosted side.
I’m not sure how easy it is to replace in this model, sometimes there is a latch or screw which holds it in place from below, otherwise you need to get to it from the top removing quite a lot of the electronics and the prism unit to get to it.
Unless the scratch bothers you it maybe best to leave it alone. Otherwise, you’ll need to find some with good camera repair skills locally or ship it to Nikon to fix.
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