  We study the magneto-photoluminescence of an optically trapped
exciton-polariton condensate in a planar semiconductor microcavity with
multiple In0.08Ga0.92As quantum wells. Extremely high condensate coherence time
and continuous control over the polariton confinement are amongst the
advantages provided by optical trapping. This allows us to resolve magnetically
induced {\mu}eV fine energy shifts in the condensate, and identify unusual
dynamical regions in its parameter space. We observe polariton Zeeman splitting
and, in small traps with tight confinement, demonstrate its full parametric
screening when the condensate density exceeds a critical value, reminiscent of
the spin- Meissner effect. For larger optical traps, we observe a complete
inversion in the Zeeman splitting as a function of power, underlining the
importance of condensate confinement and interactions with its background
reservoir excitons.
