how the beep beep beep do i get rid of an oil slick at top of tank

[Milad]

so half of the top of my tank has an oil slick on it. it is basically everywhere that isnt rippling. so my question is how the heck do i get rid of it and why is it showing up?

its really starting to get annoying. i dont want to skim it out for it to return. Ideally i want to get to the root of the problem.