It's a while since I've seen a "what is this" post on here. So just to put that right, can anyone tell me what the substance is in this picture. I often see it around the odd flight pond, sometimes a bit away but whether this is coincidence or not, I don't know. It's jelly-like, perhaps the consistency of the jelly one might find in a pork pie but more opaque. I get the feeling I should know the answer to this, but in fact, I don't. Thanks.