While out shooting yesterday, I noticed a mark through the viewfinder that I haven't noticed while I've owned my D3100.

I attempted to clean the mirror with my brush, but can't seem to budge it, it almost looks like a really small hair.

Here is a picture of it, I've circled the mark (there are a couple of other bits on the mirror that need brushing again, but mainly worried about the circled one).

IMG20170815104648 by andrewbensch, on Flickr

I know that the sensor is clean, as it only shows up through the viewfinder, and not on the finished photo.

Can someone confirm for me that it is a piece of rubbish that has perhaps has worked it's way up under the pentaprism (?) glass?

Thanks everyone