Certified GKH Gardening Expert
It can be very hard to identify pest grasses, but it is probably a Juncus species. You can take samples to your local extension service to get correct ID. This link will help you find the closes one: https://www.gardeningknowhow.com/extension-search/
I can recommend a way to start killing it off in the meantime. This will not hurt your soil, so that the target grass can take back over. Pour boiling water over all patches off this, and it will quickly kill it. Keep any remaining grass cut before any seed heads appear.