The central metal ion in ...

The central metal ion in plant chlorophyll is Mg++, so you should prefilter the water to remove cells before adding any oxidant.    Your statement is somewhat confusing since you refer to organically bound Mg++  and soluble Mg++.  The former would be mostly removed by the prefiltration (conventional treatment with sand filter) and the latter would be removed by an  NF or RO membrane. Certainly the UF would clog rapidly if the cells have not been removed.   Algae grow under conditions of slow moving shallow water, warmth and sunlight plus nutrients. Reducing algal growth in source water  is primarily managed by reducing the nitrogen and phosphorus nutrient input. Sometimes aeration of a small body of water will also control algae growth. The other concern about algae is the possibility of cyanotoxins and geosmin and isoborneol production. That's why it is essential to filter out the cells before adding oxidant which will lyse the  cells and release toxins.