I used to work on Argon Lasers a long time ago for shows. We used Acetone to clean optics and it worked miracles!.
Is it safe to use Acetone with the optics in the GlowForge?
The way we used them was with a special optical grade swab, we impragnated it with acetone and then a gentle swipe of the swab while rotating thru the optic in only one direction with little pressure. It did wonders to grease and dust.