It looks like soft rot, which is a bacteria. Most experts say it is difficult to control but you can try several things:
Prune away the affected parts
Repot it using new soil (sounds like you recently did this so maybe try the other two)
It might be white mold, a.k.a powdery mildew, it may depend on how much you water it. Make sure that the soil completely dries before watering it again.
If the plant is getting too much water, the leaves may get soft, soggy an may even produce white mold.
Let the soil dry out and leave it dry for a few days to a week and the mold should die after a while. Or,...
Short answer: yes you can.
Long answer: LECA pebbles comprise of many small bubbles (which are formed via the extreme heating of the clay). Hence, when you crush them, you get a similarly porous surface. From personal experience, I have crushed LECA pebbles and used them to lighten soil for saintpaulia and succulents. There are a few practical considerations ...