You have to be careful. Some fish such as loaches will be burned by salt...
My understanding from Nav's old threads is this:
Miracle (or its far cheaper equally useful competitor - Mineral) Mud works by containing a number of the macro and micro nutrients used by macro algae when growing.
It facilitates faster growth of macro and thus helps in nutrient export. Its usefulness when combined with skkimmers is up for debate with one school of thought saying it is fine (especially with undeskimming) and antoher saying the skimmers remove a number of the nutrients the mud brings and effectively cancels it out.