Mold: that white or blue or gray mycelia mat is an aerobic organism, that means you could find it in the damp residue on the bottom of the carboy. On a full carboy you could find it at the air/ wine interface. The oxygen requirement keeps it from growing while the wine is out gassing and while properly topped off.
In the scheme of things all grapes will have some mold spores on them which means all must will have some spores in it. Wine is a preservative system, a synergy between pH, alcohol, anaerobic environment and additives common in the last two centuries as meta.
Mechanically removing it with cold water should be enough to prevent an issue when the carboy is next used. We have neat chemicals as soap and meta and Oxiclean and even bleach that add additional protection, go ahead if you feel a need to sanitize,,, but won’t remove mold spores from the grapes in the field, the preservative traits of wine keep mold from being expressed. If you use natural cork bleach can be an issue and in lots of 100,000 guaranteed a few will be found. If you use synthetic corks the bleach issue doesn’t occur.