Appears to be hair algae . But it not really an algae. It is actually brown Diatom algae is not an algae, but a single-celled organism that appears in an aquarium when there is an abundance of Silicate. It will usually appear in the first few weeks of setting up a reef aquarium. Time is needed to let this subside as well as two of the following snails: Nerite Snails, Cerith Snails, and Margarita Snails can all help to reduce Diatoms. The best course of action with Brown Diatom Algae is to let it run its course, but your aquarium may look gross and you want to help speed it up.
Having a strong, random flow in your aquarium is not only going to be beneficial to the coral you may eventually purchase but it also helps to control Diatoms and other blooms like Cyano and Dino’s.
Good, strong, random flow prevents detritus and Diatom cells from settling on the sand and rock. Keeping the material suspended in the water column provides it a better chance of it passing through your filtration and being removed from the water altogether.
Detritus being allowed to settle and break down will also introduce Ammonia, Nitrates, and Phosphates into the water which just provide more food for your Diatoms and Nuisance algae.
As a last resort, you can use liquid vibrant which attacks virtually every algae under the sun, It should in conjunction with reduction in white light intensity knock this right down.