Aquarium Oily Film On Surface. Removing the slick is only a temporary solution; Yes, it is a biofilm or called scum.
Anyway, i have a light white, sorta oily film on the water surface. Actually, this is a pretty common thing in our hobby and i do not think that there is a single aquarist who has never had it in the tank. Lack of surface agitation allows the proteins to build up to form the oily film.