See how your sandbed in this closeup above isn't crudded with waste down into the cross section? Nice...your sandbed isn't the cause, the algae has set some holdfasts on the grains and any time the algae is fragmented in that area it simply keeps coming back, that was its anchor point and not the rocks.
Recent alt threads in the reef forum show sandbeds full of waste, the keeper having years long issues with topical invaders situated above rotting waste in a sandbed, and then six pages of denying the link between the two, your tank is new and still able to be guided pretty easily without waste issues being at the heart of the issue. Be sure and preemptively clean that sand as the months go by, keeping waste ejected out of it and a low fish bioload in the tank relative to what work you are willing to do with the sand. Right now it's in clean start phase and a little algae got some holdfasts linked into the grains.
Rocks are a little easier to treat but you don't have a tough invasion at all. When the tank is stocked with some diverse grazers this could be lowered or eaten up no prob, and until then you have to hand guide it, a few options. From most thorough to least work:
1. We (I) am on a full tank cleaning bandwagon as of this year onward because it works. We know that it is no longer true that deep access cleaning causes problems in reef tanks, if you do things in order, it solves problems. but since work is required and people want to not work, they will dig in heels, refuse to access the sandbed as needed, and begin dumping chems into the tank in further seeking of non work and quick payoff. Thorough cleaning here is simple but work oriented, take rocks out of tank into holding bucket or even your sink with saltwater in it to hold the rocks wet.
2. Hand remove your larger portions of algae and simply peroxide dose the tank per the details in the pest algae challenge thread down below this thread, tank dosing anything will work but it's again a non thorough option and thorough options have guaranteed outcomes and train us to act, not hesitate. You can also spike with tech m as mentioned above, these are the no work dump in tank options everyone with algae issues wants to use. Since your tank isn't heavily stocked really any option is ok to try, and nothing beats a real full access cleaning.
The challenge thread explains why the thorough cleaning is good for your sandbed, not bad for it.
B