Film on the water

Hope this is the right place to post this. Last night I checked my tank and when I lifted the lid, there was a thin kind of film on top of the water. Not an algae film, just a very light slighly white film. I never noticed this before. I recently added a amazon sword plant. Has anyone seen something similar? I did a 20% water change two days ago and the film was not present. Last night I changed about 10% hoping to get rid of it. The water change got rid of most of it but I would like to know what caused it and if anyone thinks it could be a problem.