I think I read somewhere that "hidden agenda" has two parts. The part that's visible, and the agenda (which is hidden by the visible). Is there a term or word for the visible part of a hidden agenda?
The "visible part" that is hiding the hidden agenda is a front.
3 [in singular] An appearance or form of behaviour assumed by a person to conceal their genuine feelings.
‘she put on a brave front’
‘Some people can put up a front and pretend to be someone they are not.’
Cambridge Dictionaries defines ostensible as:
appearing or claiming to be one thing when it is really something else
In your case then there is the ostensible agenda and the hidden agenda.
There isn't anything about hidden agenda that implies a visible part. But you might need to provide some explanation for actions taken in pursuit of said agenda on a case by case basis. In which case you need cover.
a : something that conceals or obscures under cover of darkness
b : a masking device : pretext The project was a cover for intelligence operations.
"Cover." Merriam-Webster.com. Merriam-Webster, n.d. Web. 22 Jan. 2018.