Sound to me like the water retaining gel that you can mix with compost to stop your pots drying out. You normally don't see it but when there has been a lot of rain it sometimes shows up on the surface. Some compost comes with it already included, or you can get it in tubs to mix with the compost your self. We always put it in our hanging baskets.

http://www.gardendirect.co.uk/supergel-w...9d7rdr9524