According to your xml, you are using the Support Library. Therefore, you need to retrieve your item with this following:
MenuItem searchItem = (MenuItem) menu.findItem(R.id.action_search);
SearchView searchView = (SearchView) MenuItemCompat.getActionView(searchItem);
Then, you will be able to change the text of the hint with searchView.setQueryHint()
and even its color with setHintTextColor
.