If the Playasax is in reasonably good condition it should be an easy
task to remove the mouthpiece.  Carefully 'jiggle' it whilst pulling
upwards gently.  You should be able to feel slight movement whilst
'jiggling' and, as you do so, it will slowly lift out of its engagement.

It may help to soak the joint with penetrating oil first.  Before
reassembly, clean the male and female parts of the joint to prevent
future jamming.

The harmonica fits between the rear face of the tracker slots and the
back of the tube, and seals to the tracker with a thick fibre gasket,
which keeps it airtight and fixed in position.  At the rear, it fits
between two raised tracks cast into the body, so it should be impossible
for the harmonica to turn, unless the gasket is totally unserviceable.
On mine, it had slipped longitudinally, which was easily rectified.
Make a new gasket if in doubt.

If the Playasax is very badly corroded then I suggest you follow
Spencer's advice and cut it apart at the neck.

Good luck,
Nicholas Simons, GB