What is this brown mud stuff in-between my vagina?

It sounds like you may have a bacterial infection. If you are washing regularly as you describe, the brown, smelly discharge would not continue to appear. Despite not giving you symptoms (other than the smell) bacterial infections can be very dangerous. Some can damage a woman's ability to have children in the future, or if left unchecked, can make you very sick or die. My best recommendation would be to see a gynecologist about it so that he or she can determine what the issue really is. You will need to be honest your whole health history, especially about whether you have been sexually active and whether you had protected or unprotected sex. The doctor is not there to judge you beyond determining which are the correct tests to run. If it were me, I WOULD recommend getting each of the sexually transmitted disease tests that are available if you have been sexually active at all. The doctor will likely take a culture of the brown discharge, as well as take cultures from inside of your vagina near your cervix. Each of the tests is virtually painless. The doctor may also take a sample of your blood to do additional testing for infection. Rest assured, most times problems such as these can be resolved with a simple prescription. It just needs to be the correct one - so don't try to "treat" yourself. Seeing a medical professional is the best way to go.