So, doesn't "jakie" mean "what" when asking for an adjective, and "Co" mean "what" when asking for a noun? If that is true, why is this not "Co jest haslo"?
The shortest answer is that if you have some noun phrase and you're asking something about this noun, you use a form of 'jaki' (it's adjectival 'what'). If there's no noun phrase in this sentence, like simple "What is this?" or "What did you eat yesterday?", you use 'co' or one of its forms depending on the case needed.