To Leeleeah (Marine Lover), Diatoms are often found clinging in great numbers to filamentous algae, or forming gelatinous masses on various submerged plants. If stones in the water have a brown, slippery coating, you can be sure of diatoms. Sometimes the brown coating on sticks and stones is so abundant that it streams out with the current. If rushes and stems of water plants have a brown, gelatinous coating, you are likely to find millions of specimens of the same diatom. The surface mud of a pond, ditch, or lagoon will always yield some diatoms.
The brownish stuff on the rocks and clings to the glass of your DT my friend are diatoms. They make your tank look dirty. It comes from having excessive Phosphates in your tank. As people have suggested, I need to get some cleaner crews, i.e. hermit crabs, snails, starfish and the lilkes.
I've had these diatoms in my tank for about 3 weeks now.