i've been looking into the use of mud as i just bought an HOB fuge.
mr.X used miracle mud and said he saw no benifits to it and he found piece of red brick which lead him to believe it was dirt from soemones backyard.
others have said it helps ad essecial elements to the water that help benifit corals and feed calerpa or chaeto which does all the nutirent exporting. in a way i can kind of see how it might help replenish elements as it is similar to earth and once worns and insects decompose it the earth replenishes elenents to help grow plant life, but i haven't found any factual articals that varify its benifit