Wow, never seen anything like that and I find great fascination with any kind of decomposition entity, especially a cool fungus or mold, so have some basic info to work with though not much. Usually they're in the shade, or at least in mulch, as I'm sure you know. Almost looks like some non-traditionally shaped kind of mushroom? Those do show up in full sun. I wish I could see it more clearly. What does it feel like?
As a matter of semantics, I don't think slime mold is the right term, or describes what's going on there - if the grass is really dead. According to Clemson, the grass would be OK under a slime mold. Are you sure the grass under there isn't just dormant still? Most of ours is, the 'main grass' I guess, but we have an old, wild lawn with probably 100 different plants. You probably know what I mean. But hey, maybe it's possible grubs or a fungus killed the grass, then coincidentally slime mold showed up?
http://www.clemson.edu/extensi...
Any standing water issues? I feel like I'm asking questions you probably already have, but typed anyway in case not, and curious right with you.