I claim the ornamental design for an slider clip for mounting wall panels unto a track mounted onto an existing wall, as shown and described.
FIG. 1 is a perspective view of the slider clip for mounting wall panels unto a track mounted onto an existing wall, showing front, left and bottom sides in accordance with the present design;
FIG. 2 is a perspective view showing back, right and top view thereof;
FIG. 3 is a left side elevational view thereof with the right side elevational view being a mirror image thereof;