if it's red slime algae then your cause is a lack in flow. Buy some decent powerheads and that should help take care of it. Another thing that can be done is to turn out the lights for 3 days solid. Then on the 4th just run actinics on the 5th you are safe to run all the lights. That also works well. It is a very natural way to help with it. There are cloudy days on the reef so it won't hurt anything. I have an all SPS tank and I do it from time to time just to give the tank a rest.