Just add an AC 300 to the sump. It will suck up the poop and the cleaned water will go right back into the sump and then get pumped into the main tank.
I had the same thing in my sump, a layer of brown goo in the bottom. I added an airline with a 12" airstone to try and get some turnover of water within the sump. Maybe putting a small powerhead to add some current would help.