  The enigma source, RX J1856.5-3754, is one of the so-called dim thermal
neutron stars. Two puzzles of RXJ1856.5-3754 exist: (1) the observational X-ray
spectrum is completely featureless; (2) the UV-optical intensity is about seven
times larger than that given by the continuation of the blackbody model yielded
by the X-ray data. Both the puzzles would not exist anymore if RX J1856.5-3754
is a low mass bare strange quark star, which is in a propeller phase with a low
accretion rate. A boundary layer of RX J1856.5-3754 is suggested and modelled,
from which the UV-optical emission is radiated. Free-free absorption dominates
the opacity of the boundary layer, which results in the opacity to be high in
UV-optical but low in X-ray bands. The star's magnetic field, spin period, as
well as the accretion rate are constrained by observations.
