the reason the cubes form algae is because of the constant spray cycle.
reduce the cycle to one or two short waterings(about5 mins.)each. Rockwool holds a lot of moisture,and if the cubes are are soaked constantly,the roots will take too long or never grow out at all,let the cubes dry slightly in between cycles, so the cutting will be forced to seek water,hence,the roots will develop and the algae will be kept at minimal growth,hydrogaurd by general hydroponics will keep the gunk out of the system,in a NFT set-up,rockwool should just be used as a anchor for the root system to take hold,use small cubes and surround with hydroton....bonbon