your nitrates are high enough to create a bloomlike this If you arent using RO water then i would suggest you have phosphate levels too. Algae like this will thrive on both nitrates and phosphates.
IF you are using RO water then the remedy is to increase waterflow, add more cleanupcrew like snails and less food. each time you do a water change simply syphon away the algae and then add fresh made water.
If you arent using RO water ten you could well have a problem as the fresh water you are adding might have the food that the algae is requiring. I would still increase water flow and add alot more snails.
You must also remember that you tank is young and still maturing. It will take a good year to mature fully and have stable conditions etc. The diatom blom you have (it looks like diatoms) are feeding off the silicates in the tank which all new tanks have.. this is only a short phase for the system and soon you should find higher forms of algae out competing it. If you can encourage macro algae to grow or coraline algae then this should outcompete the nuisance algae for nutrients you might have in your system.
Like all things however, dont panic and rush things... Good results take time in a marine tank, only disasters happen fast. Give your tank some time and be patient.. it will reward you in the end.