It's bacteria. It eats carbon, water, and sunlight. It's generally present. It prefers slightly odd spectrum sunlight, ergo when your bulbs are burning out of date, here it comes. Also if you have sunlight hitting your tank at some part of the day.
It doesn't really hurt anything per se, gives off oxygen, but in large amounts, it can smother corals and prevent them from feeding.
To get rid of it, lights out, tank shrouded for 3 days (4 if really awful) a good skimmer working to remove waste, and then 1 day of actinic-only to let the corals wake up gently. (they can sunburn, otherwise.)