I claim the ornamental design for the headphone pillow slip cover, as shown and described.
FIG. 1 is a rear perspective view of the headphone pillow in use;
FIG. 2 is a top view of the headphone pillow slip cover;
FIG. 3 is a left-side view of the headphone pillow slip cover;
FIG. 4 is a front view of the headphone pillow slip cover;
FIG. 5 is a rear view of the headphone pillow slip cover;
FIG. 6 is a bottom view of the headphone pillow slip cover; and,
FIG. 7 is a front view of the headphone pillow slip cover.
The broken line illustration of environmental structure in the drawing is not part of the design sought to be patented. As such, the over the ear stereo headphones are shown in broken lines for illustrative purposes only and forms no part of the claimed design.