Diatoms are a brown filmy substance on the glass or sand. Not actually an algae, but a colonial single celled organism, older than the hills. They usually appear first as dots on the glass.
They will usually eat themselves into oblivion, unless your adding DT's or another phytoplankton.
If they become a problem, add rotifers. They will compete for the same food source, and the rotifers will win. Plus the rotifers are a better food source for your tank than diatoms.
HTH!