I have done such forbidden stuff for years. I would do the following: remove the screen and hold it by the sides. (Have a very soft camel's hair brush handy.)

Now go to the kitchen sink and squirt a drop of dish liquid on the screen. Then wet the brush and brush the screen on both sides. Rinse the screen WELL and then, finally, dip it in an ounce of clean water that has a drop of wetting agent in it. Then use a hair dryer to dry each side of the screen.

The dust can also be on the underside of the prism. Before placing the screen back blow heavily on the bottom of the prism (but make sure that your lips are VERY dry so no saliva goes with your breath!) If dust is still on the mirror use a VERY clean DRY camels hair brush to brush it clean. I have even CAREFULLY cleaned the bottom of the prism with a tiny, clean tissue with a drop of Windex. (Watch not to touch the damn black baffle that prevents the mirror from banging up to hard. That black stuff is sticky and can cause all sorts for problems, especially if some gets on the screen.) This all is not a big problem if you are careful not to scratch the screen. The screen is very vulnerable to scratches because it is soft plastic, but what I have said works and will cause no problems. - David Lyga