Yup... rust colored spotches that eventually turned into a rust colored mat. Strangely enough, as they started disappearing, they started disappearing in circular spots too.
Prime will not remove silicates. Only thing I know of that will remove silicates from aquarium water are some of the Phosphate removing media/sponges. Specifically, GFO
(granulated ferric oxide) removes both phosphates and silicates. But my experience with that is strictly saltwater so I'm not sure if it does the same thing in freshwater.