Could just be an accumulation of small particles of uneaten food, decomposing organic matter...etc that are too small to be trapped by your regular filter media. You could try getting one of those micro filter insert pads and see if that helps. If this helps, it may just mean that you might need more water circulation in your tank. Adding a small powerhead will help things stay suspended longer and get sucked up by your filter and not settle on the tank floor.