What is the condition of the nutrient and what is the condition of the roots. If the roots have brown slimy patches or you can see that mold type material in the nutrient then you could be in need of a system clean. If it's only on the top of the rockwool cubes then I would suggest that it is not that bad for the plants as its just something that is visible because its in the nutrient that has wet the top of the rockwool cube.
If that white stuff is just on the surface then try mixing up a teaspoon of baking soda in a spray bottle to a pH of 6.0 and spray that area alone with a light spray making sure it only goes on the rockwool and stem and not on the leaves. Let me know if you see more mold or slimy stuff in the reservoir or on the roots.
