It's due to the lack of water agitation (movement) at the top of the water's surface. Since the water on top does not move, it just builds up protein. I haven't seen it harm my betta in anyway but I plan on moving him to a 10 gallon with an Aquaclear 20 HOB filter, possibly baffled.
I perform a 50% water change in my 5 gallon betta tank once a week. Even though I have an Aqueon Quietflow Internal filter and do weekly water changes, it is completely submerged in the tank. I got him that filter b/c it provides very little current which betta loves. Hopefully that answers your question.