If the mirror has any fog or residue on it, looking closely you will be able to see it. If you can’t see anything, I wouldn’t touch it.
When I was having the occluded fan grill issue the machine got really dirty inside. Once I discovered and corrected that, I cleaned it well. During the course of that task, I noticed a slight ‘fog’ on the head window. Turned out it was on the inside.
The head uses a tiny fan mounted on the bottom to induce airflow out of the lens bore to help keep the lens clean. Since the atmosphere in the machine was filthy because of the blocked exhaust fan grill, the air pushed through the lens bore carried smoke with it, so the lens, the mirror, the inside of the window and the entire air path through the head needed cleaning.
I removed the mirror, the lens, and unscrewed the window. Using compressed air I blew out the fan (holding the fan blade still with my thumb so as not to over speed it) and using a Q-TIP and alcohol, I swabbed out the lens bore, window hole and mirror seat.
The lens, mirror and window are precision optics, so great care should be used in handling and cleaning them.
Any film or fog you see is an accumulation of very fine particulate and is abrasive, if only mildly. I recommend using an alcohol in a spray bottle to blast off as much as possible before ever touching the surfaces with a lens wipe.
The other window and mirrors on the gantry are sealed and shouldn’t ever need cleaning, only the outside of the window.
That is a maintenance saving innovation of the glowforge, never having to clean or adjust those mirrors. Sealing that section of the beam path is enabled by the tube riding in the gantry.
So, yes the optics inside the head, the lens bore and the tiny fan may need cleaning at long intervals.
Bear in mind this is from my limited experience as a laser idiot, and in no way an official practice or method condoned by the company.