By default it is not possible to give the right mouse button a specific function in a datagrid. What is would do is the following: As you said you trigger off a microflow from a datagrid. You can make the datagrid (simple) multiselect, based on your selection you can give a list as a parameter to your microflow. Make sure your list is linked to an object that you can use as a context object for your popup page. You can open that popup from your microflow and then you can show the list of all objects that you have selected in the datagrid in that popup over association.
Was not sure about the solution scenario what we are expecting here there might be a work around , as an alternative idea to be edit the data grid column value at the place. We can make column editable and update the value at the place.
Hope this helps!