I've got a snake plant living in my room. I also have a gecko. A few days ago I turned on my humidifier and closed all doors/windows to raise the humidity to help my gecko with shedding. However, I've noticed that one of the leaves on my snake plant is turning yellow along the middle and bottom. I haven't watered it since, and I can't quite figure out what to do.
There is no way that one day or two of high humidity would harm your snake plant. So nice for the gecko, that humidity really helps. Yellow leaves have numerous causes, could very well be you are overwatering. Please tell us more about your plant: Where you live, is this guy in the house full time, does he have a very stable spot, have you fertilized and if so what and how long ago, how do you determine when to water? How big is this pot in relation to your plant? Raising the humidity would not hurt this plant especially for a day or two?
What soil did you use in the pot with your snake plant? Sterilized potting soil without any fertilizer added to the soil or any water holding gimmicks, sponges or gels?
Get used to the heft of the plant and pot when watered. When that plant needs watering again that heft/weight is markedly lighter. Only water when the pot feels vastly lighter than what it feels after having just been watered. Always dump the excess water from the saucer, do you have a drainage hole, that kind of thing?