You are probably seeing a growth of cyanobacteria in your tank. If you have a look at some of the active algae threads in this forum, you will see a lot of good advice on eliminating it. In a word, it's all about controlling the amount of nutrients in your tank water. Protein skimming, live refugiums, regular water changes and appropriate water flow to avoid dead areas that accumulate detritus are some of the many strategies employed to deter cyanobacteria. See Julian Sprung's book "Algae: A Problem Solver Guide" for lots more details.