Fungus/mold question

[Stugy]

So I've been waiting for several days now for my excavator clay to dry up and apparently the cork bark I buried in it started to mold and also some of the clay around it. I immediately removed the area but how can I prevent this? I heard cork bark was mold resistant so I'm a little annoyed. I'm also thinking that since I did all the clay work outside, maybe some spores landed on it. Anyways, how can I prevent this and would it be better to mix sand in the clay? I didn't mix any sand in the clay because the only has I have is very fine (the stuff you would get for Sicarius ) so I don't want that stuff getting into my DHS.