New tank?
Looks like diatoms (maybe some dinoflagellates here and there, but this is hard to tell at the current state and would be much easier to verify with a microscope).
If the tank is new you don't need to worry about it. It's part of the "ugly phase". I would recommend researching this term if you haven't already. Green hair algae will follow in a few weeks/months. You don't need to act in this case.
If the tank is at least 6 months old you might need to improve the deionization of your RO/DI setup. DI resin depletes faster than other filter cartridges and needs to be replaced. Without it you may introduce silicate into the water which diatoms require to build their outer shell. Once it is no longer a limiting factor diatoms can take over. They are easy to control by simply ensuring there is less silicate in the water. If you buy your water instead of using your own RO/DI system, you might have to test the water with a "pure water tester" (TDS meter that operates at very low levels). The water should preferably test below 1ppm TDS. If it doesn't you might have to get water from somewhere else.