Dark mildew smelling stool that floats?

Not mildew. If this is a persistent problem, you may want to get it checked out especially for fat malabsorption (Celiac disease etc) when the stool floats. If this happens only for a few days, don't be worried over it. It is unlikely to be mildew even though yeast is a usually resident in the intestines.
Fatty malabsorbtion. If this is a chronic occurrence I would definitely seek medical evaluation to rule out a malabsorbtion syndrome that is interfering with your ability to digest fat. Floating malodorous stools are a sign an inability of the intestine to digest fat.