According to their MSDS online at http://www.marsolve.com/Marsmsdsnew.htm
it is proprietary magic chemicals including "hydrogen chloride aqeuos" and something caustic. In other words, hydrochloric acid aka muriatic acid. You can buy muriatic acid at any masonry supply (and often hidden in the masonry section at Home Depot and such, only the professional brick-and-stone guys know where it is) and it is used for dissolving stubborn calcium buildups and such.
More aggressive than citric aid but readily available.