A couple of ways of getting rid of it is skimming the surface with a python when you are doing a water change, or get a dry sponge and drop it in your tank and let it float around the surface for a while and it will soak it up, or drop in a piece of paper towel then quickly remove it and repeat till it is all gone...
I dont remember everything that causes it but I do know a couple of reasons are over feeding and lack of surface agitation...