That mold is common with rockwool and is caused from photosynthesis from your light source on
the wet rockwool. You can cut down on it by using a cover over the rockwool cubes.
I also tried to reuse rockwool years ago and it didn't work well. My guess is the fertilizer salts
residue build up that is left from prier use. It would need to be flushed and sterilized before you use it
again which isn't practical.
That's why I use
grow rocks.
Ron