I've got two theories on my "new dark purp growth"
One is already mentioned. (temp drops)
2.) The same time I introduced superthrive it also got cold at night, making it harder to judge the cause. I DO believe I used too much ST to start with, but those two elements were the ONLY changes in environment, which created almost the EXACT same outcome as the pix above.

Did you recently introduce superthrive by any random chance?