Hard to say from the description - it could be mold or aphids. But older kale leaves will always get ratty. I would stop watering overhead and start watering at the base of the plants. Also kale does not need to be that warm. That might be contributing to the problem by promoting lush growth. It is a hardy vegetable and can cope with much lower temps. My suggestion would be to remove the oldest leaves and start eating the kale - it is a winter crop after all. I have had a nice picking from my outdoor curly kale this week. It was frozen crisp on the plants but cooked up beautifully.

BTW, I'd post future veg questions on the vegetable forum - it gets much more traffic than this one. Good luck.