not sure about the walt smith product, but i saw a study a few years ago done on miracle mud. they said there was nothing in it that indicates it came from the ocean. i had pieces of man-made plastic things in mine, and also pieces of red brick. IMO
it's from someone's back yard.
unnecessary to say the least.
i'm not convinced that there will be any benefit from it. at least your tank will be very cloudy for a few days/weeks.
if i was adding mud to my refugium, i'd shut it down, put the mud in and let it settle before i started the sump back up.