it is a bacteria.
If this is it you need to check a couple things
1) do bulbs need replacing?
2)do you overfeed?
3)do you have high phosphates?
4)is there good circulation and no "dead spots"
If all these are good then I dont know what it is but try to get these things right and if it is cyanobacteria it will go away. Dont treat it with an antibiotic because it will kill good bacteria as well.