just green hair algae.. Maybe bryopsis or similar..
Algae is 100% NORMAL for a new "immature" tank..
Typically you will see diatoms, green hair algae, bubble algae, possibly some cyano bacteria,etc...
Maintaining low nitrate levels will cause the algae it die out..
More aggressive techniques are
-Manual removal (scrub with tooth brush) can also be performed..
-You can even do a dip/soak of the rock in 50% tank water/50% hydrogen peroxide along with manual scrubbing..
But thats very little and typically one incorporates a "clean up crew (CUC)" after cycling to eat any algae that is forming.. A clean up crew will typically be snails of different types/crabs,etc... turbo snails/cerith/nerite/trochus snails are great.. I tend to only use 1 or 2 crabs (if any) as they can be "annoying/bullies" in a tank and eat snails and other sorts of problems.. Emerald crabs do good with algae/bubble algae and don't seem to bother snails..
Once the tank gets more mature you can even add urchins,etc.. They mow down algae usually..
For a new tank I would just add the clean up crew and see how they do at taking care of it along with water changes,etc.. to keep nitrate levels low (sub 10ppm)
If its still a problem then move on to more aggressive techniques..