Thank you for sending an image - it looks to me like sphagnum moss. What zone do you garden in? Moss grows in moist soil with poor drainage and poor fertility. Not sure what is causing an outbreak but I do know that improving the soil with organic matter or by aerating and adding a fresh layer of topsoil will improve the lawn so that it can crowd out the moss invasion. Moss is not killing the grass, it is just an opportunist that will grow where the grass blades struggle. You need to literally get to the grass roots of this problem to improve the soil - not just fertilize the soil but improve the drainage and tilth. If all this sounds overwhelming you may want to consider a groundcover or mounded shrub beds and pathways. Keep growing, Marianne Binetti