Probably a buildup of organic material, with some bacteria growing on it. I usually get a scum in my tank after I give the plants a good trim, as the cut ends "leak" a lot of stuff. It's harmless, but unsightly, and surface agitation or carbon in your filter will remove it.
BTW it's kind of the FW version of the same stuff that protein skimmers remove from reef tanks, and the foam that you see at the beach sometimes (which is nature's protein skimmer). Except in FW it doesn't really cause any problems and doesn't need to be removed like it does in a reef tank.