I'll give you a few questions that can diagnose the problem. See, algae/bacteria never really goes away. Shy of using chemicals (don't), the algae will survive. Fortunately, you remove its source of food and it will go dormant. So to do that we have to ask a few questions and assess what is weak here. What is your water-changing schedule like? What kind of filtration do you have? Your light schedule? Your powerhead output and configuration. What about your feeding schedule? Your water parameters?
Also, what you have is harmless, albeit unsightly so don't worry about it affecting the health of your critters. On the other hand, it is an indicator of either an immature or unhealthy tank. Just answer the above questions. There are some pretty smart people here that love helping out.
1. water change once a month 20-25%
2. in-sump filtration with skimmer
3. 12 hr on/off light schedule
4. not sure what the powerhead output as i got it free when i bought the used live rocks
5. powerhead on both sides of tank about 2 inches below surface pointing down towards the other side
6. i only have test kit for salinity, nitrite, and nitrate and all are in check
Tank has been up and running for about 3 months