I reckon it’s the concrete slab that is wicking the moisture up into the cob floor and breeding and feeding the mould. I reckon the long therm permanent solution is replacing the slab with gravel and or stone which means starting again. It’s a known fact that concrete slabs wick water up from the ground especially if they are on low ground. You could try to dig a French drain all the way around the slab deeper than the slab to drain all water away from the slab that is doing the wicking.
