The local axis system for surface follows the right hand rule. If you imagine the fingers of your right hand to curl and move from the first node ( 3 ) to the final node ( 4 ) following the incidence ( ie 3 to 5 to 6 to 4 ), the direction of your thumb will give you the local z for the surface ( downward in this case ). Local x is found as the cross product of global Y and local z. So local x = Global Y X local z. For situations where local z and global Y are parallel, as in your case, the local x is considered to be parallel to global X. So local x is from node 3 pointing further right. Once local x and z are established, the local y is found as the cross product of local z X local x
You may refer to section 1.6.3 of the Technical reference where the way the local axis is determined, is explained and also the Surface forces are shown. The MX moment is the moment about local y.
More importantly from the screenshot it seems like you are modeling a flexural member ( beam or a narrow slab ) using the surface elements. You should be using plates instead of surface elements for that. Surface should only be used for modeling shear walls.