Ok, may i ask what you exactly mean with "alien hard on"? I am not a native english speaker, so i may miss out on some of fine language things.
I am probably the one responsible for this joke, so I'll explain: "hard on" means an erection.
I deliberately misspelt "hardmode" as "hard-on mode" once, and apparently it stuck.
I can't say I'm terribly sorry though, it's still funny.