I had the same problem. I was so upset. I finished my
hugel last August and my chickens kept removing all the top layer soil. I finally gave up and put a
fence around the hugel. Re-topped it with soil, covered it with straw, and started planting. It wasn't long before the straw started sprouting. like you I thought it was grass until I pulled one and saw the seed. I also sought advise from the knowledgeable people of permies and got pretty much the same advise. When the (I will call it straw, because I don't know what it is) gets about 12 to 18 inches tall I cut it at soil level. This sounds overwhelming, but for me I had gourds and pumpkins planted in my hugel last fall and they have come back this spring. They have managed to smother out most of the straw, I only have a couple of little clumps at this point. I don't really want a hugel full of gourds and pumpkin this time of year, so I use it like a living mulch, and chop and drop the leaves that crowd the veggies I want on the hugel. What seemd like a disaster in the beginning is working out well. Some of the gourds have grown out of the
fence, and the chickens are eating the baby gourds, so it's a win win. I'm sure what ever you decide to do things will work out. Good luck to you.