FIG. 1 illustrates a top view of the claimed design for a mustard cap; FIG. 2 illustrates a front view thereof; FIG. 3 illustrates a left-side view thereof; FIG. 4 illustrates a right-side view thereof; FIG. 5 illustrates a rear view thereof; FIG. 6 illustrates a bottom view thereof; FIG. 7 illustrates a cross sectional view thereof taken along line 7-7 of FIG. 2; and, FIG. 8 illustrates a perspective view thereof.