On rare occasion, I ask to use a photograph from the UBC Botanical Garden Forums for BPotD, particularly when it's shows an intriguing subject. Such is the case today with this image of a slime mold from some anonymous folks in England via their garden installer, “Souren”: Weird Growth on Lawn Grass – What Is It?
If you missed the explanation on slime molds from a previous BPotD entry, see Fuligo septica for details and a comprehensive set of links. Physarum cinereum belongs to the same family as Fuligo septica. According to The Hidden Forest, this means both species contain deposits of lime (calcium carbonate), apparently a defining characteristic of the family.
Christine Baker of Iowa State University Extension writes about slime molds and backyard plants in Slime in the Yard and Garden (scroll down for the photograph that makes me believe my identification of this slime mold is correct). I was glad to have found a similar photograph of the organism covering grass blades, as close-up photographs such as the ones found on The Hidden Forest: Physarum and Discover Life: Physarum cinereum appear deceptively dissimilar to the submitted photograph because of scale.