Yes, it should be.
I doubt there's any easy way to remove it. There aren't any access points through which you could direct a blast of air (which in any case, would just blow the offending debris onto some other part of the viewfinder).
With the proper tools, it's not hard to remove the M6's top cover, and you'll have full access to the viewfinder optics and should be able to remove the debris easily. That said, I guess it's not something I would recommend due to the risk of damage (unlikely, but you never know?).