howdy.....1st ~ yes it is mold and or algae....both of which are very simple to prevent.
All you have to do is add about 1 inch worth of red clay pebbles to the tops of the rockwool and spread it out equaly around the tops of the pots/buckets or grow nets, this will prevent the tops of the rockwool cubes from getting any mold/algae.
This happens when direct light and heat is available to things with moisture in it.....such as your rockwool......the fungus starts with longstanding moisture and high humidity....the mold needs moisture to begin growing and digesting what they are growing on.....so by adding the little round red clay pebbles, you elliminated their process of ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Longstanding Moisture + Direct Light + High Humidity = Mold & Mold Growth.
I hope you find this most helpfull,
Cheer's and goodluck,
Bliss.