I put bleach on mold around my basement drain, and it grew ten times as big, turned brown, like manure. Help!?

I swept up the "sludge" that formed, sprayed it with hot water, and the drain was plugged. Used Drano, and it didn't do a thing except foam and then water came up from the drain and is still standing there. We'd had a lot of rain and I think initially the sewers backed up into my basement..but what is the brown stuff and how do I kill it? It seems to keep mutating!