I had a canon 600du which had something trapped under the glass in the same way. Luckily it was within warranty so canon replaced - if yours in in warranty I would suggest you contact them.
it got dust particles on the underside of the glass after warranty - I took it apart and it seems because of static I ended up with more than I removed.
Eventually I used a spray for eye glasses to clean but even now it's not perfect.
If you do decide to try and clean be aware that the damn glass seems to be a dust magnet !