Start a new document anysize you want (im using 500x200) and fill the background with black. Press "D" to reset your foreground and background colors to black and white
Now go to Filter->Render->Difference Clouds. Press CTRL+F 3 times to reapply.

Now go to Filter->Sharpen->Unsharp Mask.
Amount: 500%. Radius: 4.3. Threshold: 25

Now go to Image->Adjustments->Hue & Saturation.
Check the colorize box. set Hue 200. Saturation: 46. Lightness: 0

Then Filter->Stylize->Wind.
Method: Wind. Direction: From the Right

Then Filter->Stylize->Wind.
Method: Wind. Direction: From the Left

Now Image->Rotate Canvas->90CCW.

Then Filter->Stylize->Wind. Method: Wind. Direction: From the Left	Then Filter->Stylize->Wind. Method: Wind. Direction: From the Right

Then rotate it back to normal: Image->Rotate Canvas->90CW

Now go to Filter->Stylize->Emboss.
Angle: 135. Height: 1. Amount: 500%

There you have it, mud wall texture.