Is your aquaria doing well? Do you use natural seawater or have introduce some stones recently. Do you have low nutrient levels - especially N?
I think that I know what it is - a very small bacteria plankton using photosynthesis. Food for many filtrating animals and IMO a very useful organism in a reef aquarium. But the family name will scare your pants off. The colour is typical - this blue green colour. And it was also the old name for this family - blue green algae - more known as cyanobacteria. But calm down this is probably a plantonic species that is very important in the seas foodweb.
However - because its difficult to know exactly which species you have - just be careful - there is nasty ones in that family too
By the way - I have start to dose a species of these bacteria plankton to my aquaria - However - not a nasty one.
Sincerely Lasse