The problem with our Red dirt is that it contains a red oxide that stains and is hard to remove.
I would presume it would be similar to your red clay like on a baseball pitch. It is just hard to remove with just a spray and walk away method of cleaning.
Please do not get me wrong, I am not trying to change the way the World turns and SH does and always will get rid of mold, "fact" my problem lays with the dirt side of the cleaning process. When you clean a roof and when you walk away the roof has to be clean, that means the mold and the dirt has to be gone.