It's fish waste by the looks. Depending on how the water flows in your tank fish waste and other debris can build up in one place, usually called deadspots. It's not good letting it build up, you need to vacuum your gravel. You could also look at a powerhead to help with water movement. They will help minimise the build up of gunk by blowing it to your outlet into your filter. You don't overfeed your fish do you, that does look to be a fair pile of fish crap to me.