several ideas spring to mind....but i don't know what i am doing yet.
Take with a MASSIVE grain of salt and suspicion.
1. It's normal and won't hurt anything.
2. Rockwool is to wet...it's suppose to be damp, not saturated.
3. Algae needs light to grow...place a sheet of panda plastic (with a slit cut in it for the stem) white side up over your cubes.
4. Perhaps better air flow in the general area of the stems could help.