more than likely the latter cyano, its a proble with alot of tanks theres lots of ways to get rid of it personally pulling as much of it out you can or vacumning helps so does a large efective skimmer and water changes and reduced light.
it will pass if you keep everything in check i get outbreaks now and then but it goes away..but unlike diatoms it needs a kick in the but to get going
you should add a description of it or a pic for us to be sure its cyano
also water parameters/livestock/tank info
cheers