Well it could be a protein film or bacteria film. If you have standing water this will usually occur. So a filter that gets the water circulating will get rid of that. Just make sure not to leave food in the tank after your done feeding because that can contribute to a film and feed bacteria. Also, if you notice that one of the decorations you have is peeling in any way, remove it immediately because it could be the culprit, releasing harmful oil into the water.