There is no word 'cya', or - if there is - neither the OED nor Chambers has ever heard of it. There's 'cyan', which is a greenish-blue colour with various forms such as 'cyanate, cyanide' etc. Is that what you mean?
It's not a recognised word yet, but probably will be in years to come. It's a shortened version of See You (as in goodbye), used primarily by texters who want to save on characters entered.
I used to object to this kind of corruption of English, but this kind of thing is more a form of code, like shorthand, to enable faster communication.
I believe children are even allowed to use this rubbish in English essays nowadays! Were I to go back to school, I'd insist on answering teachers' questions in semaphore and would submit compositions solely in morse.