If you have a white powder or white substance on the surface of your hydroponic rapid rooter, I have the solution.
According to www.southeasternhydro.com, this stuff on the rapid rooter is a fungus.
To remove this, you simply use an eyedropper with hydrogen peroxide solution 3% and put it onto the plug surface. The hydrogen peroxide will not harm your seedling, and it is an environmentally safe disinfectant alternative to bleach.
After a couple of treatments, carefully rinse off the rapid rooter with warm water to remove the fungi spores.