Get used to it. It's probably a piece of lint or thread or even hair, but it won't effect your photos. It's relatively common for things like that plus dust to get into there. Unless you can easily remove the focussing screen don't mess with it.
What you are seeing is probably some sort of debris in the viewfinder. You can try blowing air into the side of the viewfinder or up into the viewfinder through the front of the camera using a bulb syringe type of blower. If it is still there in the viewfinder after that, it can be removed by our Factory Service Center.
To start your repair process, you'll need to complete a Repair Request on our website.
If this is a time sensitive-matter, additional support options are available at Contact Us.