As far as the mold is concerned, it can probably be wiped off.  It
doesn't seem to grow back once you get it to a normal environment.

Musty odors can be really awful.  Most odors are easily removed using
a very simple method.  Put some barbecue charcoal briquettes on a paper
plate and set them inside the piano and close it up.  If the odor bad,
use some above and below the keyboard.  Leave them there for a few days.
If there is still some odor evident, replace with some fresh charcoal.
Its pretty amazing how well this works.  I have always used fresh
charcoal from a new bag.

Bill Chapman,
in cold Palm Springs, Calif., where climate is really changing.