Top Fin pro 50

I have had this filter since last fall and it does ok for my 29 gal. It does have a tube extended from the inflow (with sponge filter cover) that is called „skimmer“ yet it doesn’t really explain in the instruction papers. Yesterday I noticed a little film ( probably from the fishfood) on the water top and realized that I dont really know what this skimmer should do. I don’t even know if the top, the swivel thing, should be under water or 1/2 and 1/2 or if it should always turn. I have called PetSmart (I read it is their brand), talked to 3 people, incl. manager, but no one knew. I have googled and it always talks about an adjustment valve on other models. I am aware of it not being a total necessity for a freshwater 29 gal but figured it couldn’t hurt because I thought it would „disperse“ the top film if it appears.
Could anyone here please tell me how this „skimmer“ is suppose to work?

Skimmers pull a small amount of water from the surface of the aquarium. This prevents a protein scum from developing on the surface of the water. They are mostly used in saltwater tanks, in most cases for fresh water aquaria they are not needed. Most PetSmart staff wouldn't have a clue on how to setup or use them. You likely have to set the height so that some water from the top flows into the skimmer. I am not familiar with this specific top fin product so cannot say more.

Thank you..I ll lower it a bit and see what that will do