How often you do water changes and how much? Even though you have filter you need to do weekly water changes with gravel vacuuming and filter swish/wash :)
What you described i think it just fish oil. I have 10 bettas and somehow 2 of them has this film also. If i touch it it breaks a part - white stuff . And if you don't touch it kind of cloudy but only on the surface of the water. The fish i am talking about one is 4 years old and another one about one year and 8 month old. So it definitely doesn't effect them. I think that what it is. I don't have filters and i do 100% water changes . So i think you fine, but make sure you keep water super clean .