Your tank is still very young and surface scum like that is not uncommon, especially in newer tanks. What's the root cause? dunno.. too many variables. Bacteria from overfeeding and/or organic compounds breaking down? Something in the tap water? Poor maintenance? Poor air quality? Do you spray air fresheners, perfumes, deodorant, anywhere near the tank?
I had similar scum in a 5 gallon I set up a few months ago.
What worked for me : swapped out the filter with a bigger one I had kicking around to create more flow and surface agitation, did a couple extra water changes and it was gone in a couple weeks. One of those Eheim skimmers would be cheaper then a new filter.
What you can try:
More surface agitation is needed to help break that layer.
Raise your spraybar so it's breaking the water surface, a little ripple isn't going to cut it.
Add an air stone to help break up the scum.
More water changes with Prime. I wouldn't use that Stress Coat for regular water changes, maybe in your case it's the aloe or something else in there reacting with your tank. Stress Coat is known to cause excess foaming in salt water skimmers, maybe under certain fresh water conditions it causes surface scum. Some people have reported that they get an oily looking film on the surface from using Stress Coat in fresh water tanks.