Filter Sponge

The brown slime on the surface of the sponge is decaying fish food and bits of plant. If you had fish it would also be fish poo. You can wash it off, gently just in case you do have some bacteria started, in a bit of water taken from the tank. When you have fish and are doing regular water changes, that's the time to clean the filter media, in the water you've just taken out. As you aren't doing a water change, take out just enough water to wash the sponge and put a bit of dechlorinated tapwater in to replace it.

The bacteria live in the biofilm which is a thin layer on every surface in the tank. Though the bacteria prefer the filter as there is a constant flow of water bringing food and oxygen, and it's in the dark which they prefer, there will also be some on the glass, the substrate and the decor.

When you have fish and are doing regular water changes, that's the time to clean the filter media, in the water you've just taken out.

This is a good tip this for any fishkeeper - always use tank water when cleaning your filter media and don't be tempted to clean the media too often.