Looks like green hair algae, did you buy this set up used? Are you using ro
water? It looks like LR
but can't tell us it LR
or base rock? You have a couple of of options assuming the tank has no livestock yet.
1 find the cause ( most likely high phosphates ) and fight the fight with gha. I had a case in my first 20g and it took about 8 months to finally beat it.
2 remove all the rock and sand and drain tank. Do a peroxide dip/soak of the rocks and scrub the gha off. Soak the rocks in ro
water for a couple weeks after changing out the water a couple times a week. If it is a used set up and you didn't replace the sand replace that before you start the tank back up