This looks like a diatom bloom. It usually happens after the first cycle of the tank....mine just did this as well. There are things that you can add to your filter to remove the phosphates (which add to this outbreak)...too much lighting can also be an issue. Also, You can try algaecide....just make sure it is safe for marine tanks. ....then invest in a "clean up crew" (snails, crabs, etc) that eat this as it blooms.