Mildew, I think -- I was told it can appear on cool nights when it gets below 65 after 9pm... You can spray for it if it bothers you. But as soon as your nights get warmer it will go away. I always rub the spots I find between my fingers and forget about it, unless it gets bad and appears everywhere. Also, you can help by not watering late in the day while the nights are still cool.
The problem I have with humidity is that when it's really humid I feel like I can't quite breathe. It takes getting used to, but I realize you can. When my sis lived in Tampa and we would visit her in August I always felt that way for two or three days and then stopped noticing it.
