Nam, if I may, I think what's happening is that the plastic is absorbing moisture from the air, and liquifying over a long period of time. The process is very slow, as the polymers in the plastic are reacting with the H20 on a molecular level. The answer to this is quite simple.
Every week, take the figure out of your display case, and give it an ice bath. While this sounds contrary to the above, what this does is causes the porous plastic to harden and contract, which in turn resolidifies the body material, essentially "resetting" the deterioration process.
It will be important to get the water out of the figure after each bath, so you will need to throw it in the dryer on high with a pair of old tennis shoes, until the figure is dry inside and out. You will know this has been accomplished when the figure has a lavender scent like a fresh spring meadow breeze. At this time, it will be safe to return the figure to the display case for one week.
If you notice water has remained inside the figure after completing the above process, it is recommended that you dip a clean cloth into used motor oil and wipe the figure with the oil vigorously. What this does is coats the figure in a protective shell, as you rub your figure. The metal engine shavings in the oil will act as a sort of exfoliating material, ensuring your Batman will have a clean complexion for years to come.
If you still find the figure is not to your liking, one final step would be to mix a solution of 50% ammonia, and 50% bleach, and let the figure soak in that for one hour.
(Please see below for an important safety message).
P.S.
Do not mix ammonia and bleach.
This will kill you.
P.P.S In fact, it's best you don't attempt anything I've said. I was clearly talking out of my a$$.