Light green color. Thin blade. Starts out with a small area and quickly spreads.
How Do I Get Rid of My Lawn Fungus?
Certified GKH Gardening Expert
It does sound like some type of fungus, though pinpointing the exact type may be diffcult as there are many. You will need to use a fungicide on it. This article will help: https://www.gardeningknowhow.com/lawn-care/lgen/grass-fungus.htm