More then likely it is Diatoms.
This tank is pretty new and as you go forward you will notice cycles.
Diatoms are some of the first things to appear in a new tank especially in saltwater and tanks with excess silicates and high carbonate hardness.
It could be a combination of diatoms and some sort of Cyanobacteria.
Cyanobacteria feed on excess phosphate and usually don’t show up till later in the tanks life, but if a specific rock happens to have some stored Po4 it will grow cyano.
Overall what you are seeing is normal and nothing to worry about.
It’s just part of the process to establishing a saltwater tank.