It sounds as if it's either on the mirror or the focus screen... Merrely a nuisance, won't effect images.
The focus screen is easy to remove... Go buy a pair of plastic tweezers and use those to grab the little tab, after unclipping the focus screen. Be careful not to touch either the front or the back of the screen, and most especially not the mirror. That's very delicate and easily damaged.
If it's on the mirror, I strongly suggest you take it to a local camera repair shop to have it cleaned. Don't try to do it yourself. You risk damaging it and it will be very expensive to repair.
If the focus screen is damaged, it's not a huge thing... Costs less than $50 for a replacement and you can put it in yourself. The new one will come with the little tweezers, by the way.
But a repair shop might be able to clean the focus screen, too. It's optical plastic and easily damaged, too... so be careful.
There are also foam light seals/bumpers around the focus screen. Be careful to not damage those, either. But if they are getting old, they can crumble and be the cause of stuff you see on the mirror and focus screen. Again, a local camera repair shop should be able to replace the light seal, if needed.
None of this would be expensive... Unless you try to do it yourself, screw something up and end up having to have major repairs done. No, I don't recommend you take the camera apart and try to get into the viewfinder and pentaprism area. You might never get the camera back together! At the least, you have to be very careful, there are often adjustment shims used in there to be sure everything is perfectly aligned. There are a lot of parts and some electronics involved in disassembly, too. The pentaprism and viewfinder are usually pretty well sealed against stuff getting inside anyway, on most cameras. It's much more likely the stuff you see is on one side of the focus screen or the other, or on the mirror.