6 inches of mulch is way too much and should have been pulled back to allow the garlic light when it broke though the soil surface.
And yes, most straw will sprout seeds, the combine used to harvest the grain never gets every last bit.

That said, I think you can stress any plant by depriving it of light and then scalding it in full sun. I'd fluff up the straw to kill the growing grain and then use it to mulch the garlic again. But this time, don't cover the plants completely, let some light in. Then when it's cloudy, you can pull back the mulch more to allow more photosynthesis to happen gradually.
Good luck.

Thanks for sharing!! I did flip the mulch and set it alongside. The stems are greening up already and don't seem to be getting wilty. Thanks !!
- Jean in Mt