What is the thick, yellow discharge that appears after menstruation, and does it invalidate ablution?

If these discharges are mixed with blood, they are considered menses, and their duration, including any yellowish or brownish discharge associated with them, is considered menstruation, as long as it does not exceed 15 days. However, if they appear after seeing complete purity, they are not considered menses. You must wash the area and perform ablution for prayer. If they are continuous, you should use a cloth and perform ablution for each prayer after its time has begun, like a woman with irregular bleeding (istihada).