That's why I was asking if REMOVING the dust cover glass would remove the artifact, I expect it may be in the diodes output or on the diode window, but changing the lens and window can be attempted 1st.
If this became a common diode issue we would need a spatial filter with a precise rectangle cutout and a tapered edge, a pair of spatial filters separated by 20m or so would clean up a lot of the artifact/overspray.
I like to use lens flare hiders to block lens flair and also soak up any artifacts within the angle of the lens to edge of the tube, the longer the tube and the closer the diameter of the tube is to the beam the shallower the angle of overspray, just a small tube helps block a lot of lens flair and artifact, although in this case the 2nd weak beam I see here is at a shallow angle to the beam.
Lens flair is distracting to passers by and makes you stand out when lazing, just a 2 inch square of cardstock/construction paper rolled around an empty 12mm module and held with tape makes an expedient flair hider that you can slip over the standard thumb wheel of your M9 x 0.5 lens barrel or you can make something really nice, some of the Chinese greenies will have the lens recessed and that kills of a lot of lens flair.......I know this is not a practical solution to the OP narrow angle artifact problem, but it fit's along the lines of a common related issue and is a good thing to think about if you live in a metro/urban area.
SNIP