lmao typical cutie. you people are all so handsome it is unfathomable. you know what go touch some grass and make every passerby's day by the simple fact that they get to see you.
It's Cockney rhyming slang. If you want to come up with a slang term for a something, you take something that rhymes "yank → septic tank" and then cut it down "septic tank → seppo"
The only actual slur I know of is that North Koreans call Americans "white devils". Which is ironic if you know North Korea's racial demographics compared to America's.
I've mostly heard 'yanks' in English. I don't know if Japanese slang specifically has a word for USian foreigners as most situations don't really care about which country and, if more specificity than 'foreigner' is required, it's usually going to be some adjective/descriptor (loud, rude, skin_color_here, etc. in the case of people being those things).