If this brownish discharge has been seen right after the post carriage bleeding, then the same ruling of that bleeding applies to it and so you have to abstain from praying and fasting.

If there was a gap of time between the post miscarriage bleeding and the discharge of that brownish thing and in that period you were completely clean, then there is disagreement among scholars whether to overlook this discharge and thus a woman may start to pray and fast or to consider it a continuation of that bleeding.

Allah Almighty knows best.