Dust under the screen.

My personal advise is try to live with the dust or take it to an Apple Certified Provider to do the job. 90%+ of the people that attempt to remove the glass break it. If you do the job yourself, make sure you have located a place to acquire the glass and adhesive in a timely manner, as you may be without use of your laptop until they arrive.

I am certain the glass can be removed without removing the display assembly, but it is not feasible to attempt it. You increase your risk of breaking the glass and/or damaging other things. You are best off replacing the adhesive when you install the glass for it to seal properly and keep dust out.