It may be powdery mildew. I would suggest you find a quart spray bottle (like from Windex) and wash it out real well. Then add 1 teaspoon of baking soda and 1 teaspoon of Murphy's Oil soap, or any non-degreaser, non-antibacterial liquid soap, fill the bottle with lukewarm water, shake well, and spray it thoroughly on the affected plants. Make sure you get the bottoms of the leaves as well.
Always works for me.

Oh, Thank You so much!!! I am going to try it right now!! I'm so sad that I keep losing my thyme! so, I will definitely need to wash before using these, right?

Good luck with it. I'd wash anything from the garden anyway, whether or not you use any sort of spray.

Thanks for the question and the answer, I am making notes for my journal. I am trying not to use anything harmful in my garden.