Im guessing since you are in a hydro setup you are referring to a gunk that has formed around your roots. If this is the case here is the cure.
1: make sure your water is never warmer than 57 F (not freezing either though. 55 is ideal)
2: pH should be in the range of 5.6-6.4
3: You should use H202 (Hydrogen Peroxide) Everytime you change your reservoir (I use the store brand 3% at a rate of 1 tsp per gallon)
Cheers.