Its most likely diatoms/cyanobacteria and green slime or hair algae.
#1 Check your nitrate and phosphates levels in the tank. These are two key nutrients that will cause nuisance algae blooms, as well as excessive lighting.
#2 Get yourself some macro algaes to compete with the nuisance ones (chaetomorpha is a good one to get). You can get some very ornate and decorative macro algaes, which are very pretty in the tank and help filter the water.
#3 You could try running granular ferric oxide products in a fluidized reactor, which will absorb the phosphates.
#4 You could try adding active carbon to absorb the nitrates but, personally I'd stay away from it as it does add to the phosphates and in the long run it isn't solving the problem.
#5 You could also try adding a florida fighting conch which will continuously forage the sand surface.
#6 Check how much you are feeding your fish. Are you over feeding them and producing excessive nutrients in the tank
#7 Are you running a protein skimmer? These can help remove dissolved organic waste in the water and as a result, help prevent nitrate and phosphate build up.
#8 Increase the frequency that you perform a water change. This will help kick the nitrate problem if you have one. Phosphates are somewhat harder to remove with water changes apparently.
