  High resolution Hubble Space Telescope (HST) image analysis of the
MOA-2008-BLG-310 microlens system indicates that the excess flux at the
location of the source found in the discovery paper cannot primarily be due to
the lens star because it does not match the lens-source relative proper motion,
$\mu_{\rm rel}$, predicted by the microlens models. This excess flux is most
likely to be due to an unrelated star that happens to be located in close
proximity to the source star. Two epochs of HST observations indicate proper
motion for this blend star that is typical of a random bulge star, but is not
consistent with a companion to the source or lens stars if the flux is
dominated by only one star, aside from the lens. We consider models in which
the excess flux is due to a combination of an unrelated star and the lens star,
and this yields 95\% confidence level upper limit on the lens star brightness
of $I_L > 22.44$ and $V_L >23.62$. A Bayesian analysis using a standard
Galactic model and these magnitude limits yields a host star mass $M_h = 0.21
^{+0.21}_{-0.09}~ M_\odot$, a planet mass of $m_p = 23.4
^{+23.9}_{-9.9}~M_\oplus$ at a projected separation of $a_\perp =
1.12^{+0.16}_{-0.17},$AU. This result illustrates excess flux in a high
resolution image of a microlens-source system need not be due to the lens. It
is important to check that the lens-source relative proper motion is consistent
with the microlensing prediction. The high resolution image analysis techniques
developed in this paper can be used to verify the WFIRST exoplanet microlensing
survey mass measurements.
