It's just a hardened limestone mud stain confirming that the structure is very old, has an inclined portal to the outside that certainly could have collected rain, condensation,etc. that simply could have drained downward and carried some of the limestone powder dissolved into it. Why is that such a difficult "mystery" for some?
Because the stains also appear in the QC-shafts. There is no evidence that these shafts were open in ancient times.