That is diatom. It feeds off of silicates and as soon as it depleats the silicates in the tank it will die off on it's own. All new tanks go through it. As mentioned above, it usually happens as you aproach the end of your cycle. You wont need a lot for a CUC with that size tank. Maybe 10 snails and 10 hermit crab. You can get them at any local fish store (LFS) or you can order them from Reefcleaners.com. If you order them, buy for a tank half the size that you have. They send a ton. LOL