Usually the oily surface scum is organic and harmless on the most part, just looks bad.
Using a paper towel will help to rid it and then make some 100% water changes since you don't have a filter, water movement will help keep it broke up so you don't notice it as much.
If the tank is close to the kitchen the oils and smoke can cause it, oils from your hands and food can cause it.
Rotting plants is a big cause and the bamboo can create a slim and rot if more than just the roots are in the water.
Clamping can be a sign of too cold or poor water quality, I would start by stepping up your regular water changes to 50% water only twice a week and once a week make a 100% water change in that size of tanks without filtration.