NEED HELP STAT!

im 62 days into my saltwater tank and been cycleing the rock and i just started noticing red algae in my sump. Been doingmy normal 20% water changes every week with Ro water. I have one fish in the sump to help cycle the tank and i only put enough food in that i can see it eat. I just wondering where its coming from. If u can help thanks. I want to take care of it before it becomes a probelm