I have not seen that (+) icon exposed in the XML; maybe I’ve missed it or there are other methods (CSS?) that can change it. 

Usually, when we want to make a change like yours we will turn off the inline create option and add a global action instead. It shows up as a button above the table instead of an icon in the header, but the icon and the label are both editable in the page composer. The only action you would need would be a Create Row in GenericJunction. Another benefit is you can add the new row at the bottom of the table instead of the top if that fits your use case better. A downside is if you have another global option, the button above the table has a selector arrow instead of directly exposing all the global actions; this may not be obvious to a user.