It is not mud. Firstly it is not liquid, it's frozen. Secondly, it's not permafrost where it's mixed with a considerable amount of dirt, it's ice, a sub-surface

It is not mud. Firstly it is not liquid, it's frozen. Secondly, it's not permafrost where it's mixed with a considerable amount of dirt, it's ice, a sub-surface glacier, one of many on Mars, merely the largest one we've discovered so far.