Ok well I am not an expert on UV
sterilizers but I will try to help you out here. From my understanding UV
sterilizers do not eliminate - cyanobacteria (blue green algae), diatoms (brown algae), dinoflagellates (brown slime algae) or chlorophyta (green/hair algae) and that appears (to me) to be cyanobacteria. What is your tank setup, fish, how often do you feed, and how long do you leave lights on?