The other reviewer is a bit confused. Preserved lemons are made at room temperature. Otherwise they won't ferment. It's doubtful that mold was growing on your lemons in the salty, acidic brine, unless you did something wrong.
Incorrect sttep makes the lemons moldy While this recipe is very similar to other recipes for preserving lemons, the key mistake is that they ask you to leave them on the counter for 10 days rather than refrigerate them. I followed the directions to the letter; sitting the jar on the counter for 10 days makes the lemons moldy. You can't actually SEE mold, but I can absolutely taste it. If I tried this recipe again, I would not set them on the counter; I'd refrigerate them.