I think you mean the imperative form (pomóż!). If it's any help - [potrzebuję] pomocy ([I need] help!)
pomóc is the imperfective of pomagać.
pomoc is a noun.
If you're asking for help, you use "pomocy!".
oh ok, it's probably because in English "help" is also noun so I got kinda confused
Yes, as Baakamano wrote above, it is shortened from "Pokazuję pomocy," and "pokazuję" requires genitive case.
You mean 'potrzebować' (to need) --> Potrzebuję pomocy.
"pokazywać" means 'to show' and takes Accusative ;)
Also I myself would interpret "Pomocy!" as vocative, it's calling (for) help, but whether you consider it Genitive for (potrzebuję) pomocy or vocative, it doesn't change anything.