Is the buildup specific to the reservoir, or can you see it in the tubing or anywhere else? If it's ONLY in the reservoir it's most likely algae buildup, as the silver kill coils only act as a biocide if the microbes are flowing along it. If algae were to build up along the inside of the res, and the kill coil were not in any contact with it, then the algae would build up.
Flush the system, if that doesn't work mix vinegar with hot water in 50/50, about an hour or so should be enough to clean EVERYTHING out. Then flush it again, to be sure there's non of your cleaning agent in there as a wacked out Ph can have bad long term effects. Then be sure to use some sort of liquid biocide.