It is generally harmless until it costs the top of the water since bettas are then nearly unable to penetrate the surface for air. Scooping out with a cup, or laying paper towel or a cloth ontop then removing usually works. Using a bubbler or filter to break the surface is best, as still water lets it settle.
Heater snot... Not exactly sure how to explain it
just some gooey filmy stuff that is usually on the heater but can make it to plants, decor and surface of the water. It's slimey, annoying and gross