Its not a piece of grass, its a stick. It can't really move because of the barbs on it. If it really was serious, my Barn Owner would have said "Call the vet, this can't wait" But, he's obviously seen this sort of thing many times before, and treated it. If it didn't have the barbs, one of the stable hands was going to take it out with tweezers, but seeing as it couldn't come out, the skin needs to be cut to have it taken out.
I'm not worried, I have confidence in my barn owner, and the farrier is trained to do minor medical operations, such as extracting porcupine quills (and other foreign bodies) so long as they are not deeply embedded. He also does stitches. The stick is lodged just under the skin, and I saw no blood or buildup around the entry of the stick. If it had jabbed straight in (not flat under the skin), I would have called it in immediately as an emergency.