Those are Diatoms.
A common outbreak during cycling.
Diatoms come from live sand or live rock, even live rock from stable tanks that have been running for years have diatoms. They are usually limited by nutrients, when nutrients increase suddenly or during cycling of new tanks, the population flourishes and gives the aquarium a nice coating of algae.
Most hobbyists assume that diatoms appear because they did something wrong. This is usually not the case, even experienced hobbyists can get a bloom of diatoms when starting a new tank.
Diatoms have an outer shell composed of silica. To form this shell, diatoms need to feed on silicates.
The 2 main sources that silicates come from:
1) Sand. When we buy sand, there is generally a high concentration of silicates. After all of the silicates are consumed from the sand, the population of diatoms should stabilize.
2) Water. If the water that you use to top off and use for water changes has silicates in it, then the population of diatoms will have an unlimited and bountiful food source and may not stabilize.
To get rid of diatoms, you need to get rid of the silicates in your water.
Hope that helps.