From saltwater aquarium guide.
"Diatoms are microscopic cells composed of overlapping half shells of silica, which spend their lives floating in the ocean or in the sediments near the bottom.
Their silica shells, called frustules, are microscopic size and makes them hard to see.
The Diatom's iIn the ocean form a major part of the “plant-plankton” called phytoplankton, which provides important food for the “animal-plankton” called zooplankton.
These algae spread rapidly with high nitrate levels and generally are the first algae to establish themselves in your saltwater aquarium. They form a brown slime on the gravel, rocks, decoration and aquarium glass.
Diatom
As your saltwater aquarium matures, they should disappear. Scrape them away to keep a clear view of your saltwater aquarium fish when they become obvious.
Like all algae, the best way to deal with this type is by targeting its prevention. To produce their frustule and live out their lifecycle, they need abundant silicates or silicic acid.
Silica sand is a common source, so it is highly recommended that you use a limestone sand or gravel instead.
Turbo Snail
Some commercial salts for mixing marine water will also increase the levels of silicates – be sure to read the labels and before you add.
Also, try to keep invertebrates such as snails. They will roam the saltwater aquarium, walls, rocks and even the hard to reach spots and feed on algae."
