The ornamental design for the floor mat hanger, as shown and described.
FIG. 1 is a front perspective view of a floor mat hanger showing my new design;
FIG. 2 is a rear perspective view thereof;
FIG. 3 is a front elevation view thereof;
FIG. 4 is a rear elevation view thereof;
FIG. 5 is a left side elevation view thereof;
FIG. 6 is a right side elevation view thereof;
FIG. 7 is a top view thereof; and,
FIG. 8 is a bottom view thereof.
The broken dash lines shown in the drawing views represent non-claimed subject matter, and neither the lines themselves nor anything within the lines form any part of the claimed invention.