Dreamerr, my stock stem gets cloudy as well near the mouthpiece end. Golden like michaels up until half an inch past the bend, after that its cloudy, and a bit cracked up like dried dirt if you look really closely. I think that it is just the beginning of the oil buildup, particles tend to clump together more often than to new surfaces, so I think its just a matter of time before that clouding builds up into gold itself.
As to why yoshis stem doesnt cloud like yours or mine, I can only imagine it depends on the moisture content and quality of your herb, and maybe the fact that his screen is letting particulates through and its disrupting the even buildup of oil? Ah thats a bit far, but possible. I use a 325 mesh screen so nothing gets through, like yours.
On another thought, do you blow through your stem at the end of sessions to get all the little bits remaining out of the screen? (because i know i do after i dump the abv). If so, its possible the humidity from our breaths, or maybe even saliva particles are gathering in there and that is what is causing the clouding. Yoshi, any input on if you do this or not?