The dust will have little or no effect on the image. Don't worry about it beyond blowing of the worst. Paranoid people have been known to gently wash mirrors upside down with very gentle water - but it's not worth it and besides you'll never get the mirror out of that telescope (or if you do you'll never get it back and recollimated). Note, I'll take a filthy corroded collimated telescope over a clean telescope with badly aligned optics

When you lose more than 30% of the aluminium surface - that's the time for action.