so ive made my way past all the security im in front of the tower its still red and theres no press 'x' icon to hack it.

anyone know what im doing wrong?

Are these the ones you need to hack into to access the ctOS network in that area? Or the smaller ones that unlock additional objectives?
If the first one, you have to find the guard with the access code and 'hack' him to get it. Then you can access the network.
If the second one, you actually have to interact with the terminal physically rather than hack it.

I have run into a bug at one Ctos mainframe area where for whatever reason it just would not let me hack the dude who had the access code. It also happened once with an audio log. Really odd but a restart of the mission resolved it.

@adequatelyprepared: its the smaller ones that really only reveal the map, im standing right next to them as well, there just isnt any prompts.

weird that there is no prompt, it should be a Y (or triangle on PS) prompt and not the standard hack if I remember right