Planted Tank Guru
Join Date: Jun 2012
Location: Gainesville, FL
Looks like "surface scum" a protein film. I think if it were GDA the glass would be affected. I had read somewhere that protein film will not break with a finger inserted into the water. The other film, iron bacteria, will.
You can remove most of it with a paper towel. Decreasing the organics by water changes, various medias like purigen, and a surface skimmer should do the trick.