Sump pit mold

I sealed my sump pit (with pump in) with a custom Lexan lid and silicon caulking to reduce radon gas in my basement. Because most of the weeping tile water collects not in the sump pit, but a drain which leads to the municipal sewer (the sump pit and pump are really designed for backup purposes), there is only a small amount of water in the pit which sits stagnant in there and does not get pumped out. Because the pit is sealed I am getting some mold growth at the top of the pit, on the sides. Is there anything I can do short of unsealing the pit and cleaning it out? Can I put a small amount of vinegar or some other disinfectant in the pit to inhibit the mold growth?

Thanks!