The ornamental design for a lightfixturecrystalmountingarm, as shown and described.
FIG. 1 is a perspective view of the light fixture crystal mounting arm of the present design;
FIG. 2 is a top plan view thereof;
FIG. 3 is a front elevation view thereof;
FIG. 4 is a right side elevation view thereof;
FIG. 5 is a rear elevation view thereof;
FIG. 6 is a left side elevation view thereof; and,
FIG. 7 is a bottom plan view thereof.
The broken line illustration of a crystal and portion of the light fixture crystal mounting arm illustrate environmental structure and forms no part of the claimed design.