For what it's worth, a prism surface or front surface mirror can be cleaned of stubborn dust with a feather usually contributed by a duck or goose. Voluntarily or not.
I will shave the feather down so it has only one edge about 3/8" long on one side of the tip of the feather. Clean it with rubbing alcohol to remove the natural oils in it before you use it or you'll end up cleaning oil off the surface too.
If whatever is on the prism or mirror doesn't come off with this tool I'd leave it alone.