It looks like you've several types of growth in the picture - hair algae on the rocks, cyanobacteria on the sand. If the tank is new, probably the best thing to do is practice good husbandry with regular water changes, monitoring the phosphate/nitrate concentration in the tank water and keep it under control with phosphate absorbing resin (GFO) and/or carbon dosing, and wait it out. Almost all tanks started with dry rock go through phases like this - people call it the "uglies".
If the tank's been set up for a long time (>6 months), you may have a systemic problem that won't go away so easily. If this is the case, you'll want to make sure that your export methods are working properly (skimmer, refugium if you have one, GFO, carbon dosing, etc...), that the nutrient input isn't too high, such as overstocking the fish, that a significant size animal hasn't died and caused a nutrient spike, etc...
If your tank isn't new, the water nutrient levels check out as fairly low (PO4 < 0.3 ppm, NO3 < 5 ppm), the skimmer is working properly, etc..., you may have significant calcium phosphate precipitated on the rocks, which fuels GHA in particular. You can get rid of it with a fluconazole treatment, but otherwise there's nothing to be done but run GFO or dose lanthanum chloride to keep the phosphate level really low, and get some critters such as urchins to eat the algae. Eventually, if the phosphate level in the tank water remains low, the algae will exhaust the supply of surface phosphate, and die out.
