The ornamental design for a “mobile supporting member,” as shown and described.
FIG. 1 is a perspective view of a “mobile supporting member” showing my new design;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a left side view thereof;
FIG. 5 is a right side view thereof;
FIG. 6 is a top view thereof; and,
FIG. 7 is a bottom view thereof.
The broken lines shown in the figures are included for the purpose of illustrating portions of the mobile supporting member and form no part of the claimed design.