Hamza there is nothing wrong with your tank.
algae and diatoms are a normal entity in a marine tank or in fact any aquarium system . They are actually good for a tank because they remove the unwanted nutrients in the water.
They will grow anywhere and everywhere when condition are ideal for them.
Conditions are light source and food (nitrates n phosphate)
Getting rid of any one of these will eleminate the algae.
Let's start with lights , u cannot get rid of light from the main tank, u can switch them off but u will need to switch them on again .
Chemical option
Phosphate removal , use phosphate removal media like Phosban,rowa etc in their reactor.
Nitrates removal ,use denitrator , carbon source , azno3, bio sphere or any other chemical gadget n reactors.
Natural biological option.
Using algae it's self .you can make other places in your tank more ideal for the algae to grow. Eg your sump, introducing a refregerium or algae scrubber which is actually growing the unwanted algae somewhere else (in the sump) so they don't grow in the display. You just need to introduce a better and longer period of light source in the sump.
All in all , algae are a actually good for an aquarium if I can grow them elsewhere and not in the display.
Your tank is going through the cycling period , it can take week or maybe months to reach a natural equilibrium , choose a system and stick to it, be it chemical or biological. Ask questions ,surf the net , get more information on the water chemistry , etc etc.