The ornamental design for a movablemountingarm, as shown and described.
FIG. 1 is a bottom perspective view of a movable mounting arm showing our new design. The movable mounting arm is shown with a monitor (in broken lines) in an open position;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof; and
FIG. 4 is a left side view thereof;
FIG. 5 is a right side view thereof;
FIG. 6 is a top plan view thereof;
FIG. 7 is a bottom plan view thereof; and,
FIG. 8 is another bottom perspective view thereof, with the monitor in a closed position.
The broken lines in the figures show portions of the movable mounting arm which form no part of the claimed design.