Is this a new tank, less then 6 months old? If so, it's very normal. Diatoms will thrive for a while, until they use up the available free silicates, then die off. There are quite a few critters that will eat the diatoms--snails and crabs, mostly. If your tank is suitable for adding these guys, great. If not, manually remove the stuff, and it will eventually go away. Don't be too aggressive in stirring up the sand--you don't want to disturb the lower levels, where the aenarobic bacteria are trying to establish themselves. Stirring will introduce more oxygenated water, which is lethal to anaerobes.
