This is totally natural in some Hydrosystems. This Salt build up is what is left over when the Nuts evaperate away. The system that is very good for leaving salt behind is the "Waterfarm". The Salt Builds up around the Halo. But it is not big deal, just wash it off every 3-4 weeks.
We are switching to this method soon actually. Some of the best growers we have met around us are doing it this way also. Straight rockwool, some rockwool mixed with coco. Nothing but nutes. Flushing every 2 weeks.
I will be using the hugos with a coco mat will let you know how it turns out, hopefully not 4 years later tho.. Also considered the granulated rockwool but heard it came in 2 parts and the ph was a pain in the azz!! But i have seem some vidz on it and the plants looked pretty sick!! You can youtube "granulated rockwool" it should come right up. Let us know how the rockwool works out. peas