  Phase mixing of standing continuum Alfv\'en waves and/or continuum slow waves
in atmospheric magnetic structures such as coronal arcades can create the
apparent effect of a wave propagating across the magnetic field. We observe a
prominence with SDO/AIA on 2015 March 15 and find the presence of oscillatory
motion. We aim to demonstrate that interpreting this motion as a magneto
hydrodynamic (MHD) wave is faulty. We also connect the decrease of the apparent
velocity over time with the phase mixing process, which depends on the
curvature of the magnetic field lines. By measuring the displacement of the
prominence at different heights to calculate the apparent velocity, we show
that the propagation slows down over time, in accordance with the theoretical
work of Kaneko et al. We also show that this propagation speed drops below what
is to be expected for even slow MHD waves for those circumstances. We use a
modified Kippenhahn-Schl\"uter prominence model to calculate the curvature of
the magnetic field and fit our observations accordingly. Measuring three of the
apparent waves, we get apparent velocities of 14, 8, and 4 km/s. Fitting a
simple model for the magnetic field configuration, we obtain that the filament
is located 103 Mm below the magnetic centre. We also obtain that the scale of
the magnetic field strength in the vertical direction plays no role in the
concept of apparent superslow waves and that the moment of excitation of the
waves happened roughly one oscillation period before the end of the eruption
that excited the oscillation. Some of the observed phase velocities are lower
than expected for slow modes for the circumstances, showing that they rather
fit with the concept of apparent superslow propagation. A fit with our magnetic
field model allows for inferring the magnetic geometry of the prominence.
