Bacterial Growth?

[Aquavol]

I have some sort of white growth in my sump. I remove it with each water change but it seems to be coming back bigger. The center is brown. It smells like sewage. I should have taken a picture because I know this description is vague.

Does anybody know what this might be? Or how to eliminate it?

 

[Aquavol]

I should have mentioned that it grows in the darkest part of the sump and it floats.