From your description, it sounds like a slime mold is residing on the grass blades of your lawn. Slime molds are primitive fungi that sometimes appear on lawns or landscape shrubs during warm weather, especially during or after heavy periods of rain. These fungi are not harmful to the plants on which they reside. They feed on bacteria, fungi, and decayed organic matter.
Slime molds have two life cycle stages.
The slime stage, which is rarely observed, looks like a slimy growth that may be yellow, white, gray, red, violet or other colors. These slimy growths may be a few inches, or up to several feet in diameter. As soil and surface moisture decreases, the slime stage gradually converts to the spore stage. This spore stage is more commonly seen. You can rub the material from the grass leaves and will see a dark powdery material on your fingers.