What causes the newer caulk in these windows to begin growing this ‘suspected microbial substance’?
Thanks in advance! 
condensation from high humidity levels hitting the cold glass is the most common explanation
Thanks Paul,
I was trying to figure out why it’s just real bad on the caulk. The caulk is still soft and pliable (its only been on the windows for about a month the HO says)
I thought maybe the type of caulk had contributed to the quick growth.
If he dad used an actual window glaze, that wouldn’t have happened. most of the “good” caulk says “mold / mildew resistant”, but some of the cheaper styles and one called “painters” caulk (meant to be painted or covered) have some impurities because they don’t use the purification methodes the more expensive one do.
Ooops, Sorry! Your right. I didn’t look at the category this post was in and was thinking we were talking about the interior side of the window.
No worries Paul…
Talked to the buyer this am and told him that I’d clean it with soap and water, allow it to dry well and paint it to preclude reccurance of the mold. It was only found on the front windows beneath the porch area (about 4 times what you see in the picture) so it shouldn’t be too much of a chore 
Yeah, your right Mike. it’s just cosmetic. probably more of a moss type fungi than a mold. i ususaly recomend a 50/50 bleach/water mix to neutralize it and then wipe it away. then like you said, paint to prevent recurence. not enough light under there for the sun to kill it naruraly.