The ornamental design for a mooring buoy, as shown and described.
FIG. 1 is a front elevational view of a mooring buoy showing my new design.
FIG. 2 is a left side elevational view thereof, the right side view being a mirror image thereof.
FIG. 3 is a rear elevational view thereof.
FIG. 4 is a top isometric view thereof; and,
FIG. 5 is a bottom isometric view thereof.
The claimed mooring buoy is shown broken-away transversely in the various views to indicate that a specific length forms no part of the claimed design. The broken line showing of parts of the drawings is included for the purpose of illustrating use and environment and forms no part of the claimed design.