  We report an optical/UV jet and counterjet in M84, previously unreported in
archival HST imaging. With archival VLA, ALMA, and Chandra imaging, we examine
the first well-sampled spectral energy distribution of the inner jet of M84,
where we find that multiple co-spatial spectral components are required. In
particular, the ALMA data reveal that the radio spectrum of all four knots in
the jet turns over at approximately 100 GHz, which requires a second component
for the bright optical/UV emission. Further, the optical/UV has a soft spectrum
and is inconsistent with the relatively flat X-ray spectrum, which indicates a
third component at higher energies. Using archival VLA imaging, we have
measured the proper motion of the innermost knots at 0.9+/-0.6 and 1.1+/-0.4 c,
which when combined with the low jet-to-counterjet flux ratio yields an
orientation angle for the system of 74 (+9,-18) degrees. In the radio, we find
high fractional polarization of the inner jet of up to 30% while in the optical
no polarization is detected (< 8%). We investigate different scenarios for
explaining the particular multi-component SED of the knots. Inverse Compton
models are ruled out due to the extreme departure from equipartition and the
unrealistically high total jet power required. The multi-component SED can be
naturally explained within a leptohadronic scenario, but at the cost of very
high power in relativistic protons. A two-component synchrotron model remains a
viable explanation, but more theoretical work is needed to explain the origin
and properties of the electron populations.
