Carbon and phos ban reactor question

[Maridia]

I understand that carbon removes chemicals, medicine, and makes water look clear. Phosban reactor removes phosphates. I also understand that you want low phosphate if growing/keeping corals. Will these reactors help control algae as well? What makes phosphate harmful, and if anyone has any exp with reactors, is it a good investment to make? I use ro water when mixing my saltwater, but regardless of 20% weekly WCs, 2 or 3 days later, brown algae begins to grow on the glass. It's not an infestation by any means, and I clean it off, but I wanted to know if any of these reactors will aid in less algae. I have a 55g salt with 2 clowns and 2 green chromis (ie, under stocked). Please help if you know a thing or 2 in this particular field. Thanks.