@hamradio: Ahh. Of course. Fascinating how one's paradigms can mess with one's perceptions. I was convinced that was some kind of wet crabgrass, or something, though I did keep thinking that it looked rather like pond scum, but then my preconception was of a "park" environment, where (in my mind) you don't have crocs, so my brain obligingly filtered that out of what the eyes were seeing - but of course you do get crocs there. I could see it instantly after @Deozaan explained what it was. Blinkers off.
What a great shot!