Unwanted moss growth
Looks like moss in both photos.
There may be several reasons you have moss in your lawn which include poor soils, shade, poor lawn care practices, acidic soils, etc. Look at our publication regarding moss control
You can eliminate the moss by raking and disposing of. However, if you want to get rid of it you need to correct the conditions that favor its growth. See above.
Begin by testing your soil for pH and nutrient deficiencies.
Options include alternative groundcovers and some actually cultivate the moss.