Did you try to clean it?
First I would scrub it with a stiff brush, with a solution of water+bleach-but you may not like idea of using bleach (it does have short life if exposed to air, but some feel that salts in it may be absorbed into clay pot & leach out).
Or fill a spray bottle with vinegar and saturate the area. Let it sit. Approx. 80% of the mold strains can be eliminated by vinegar alone. For tougher mutant molds, try two teaspoons of tea tree oil and two cups of water. The smell will be strong and can linger a few days, in addition to being toxic to animals.
You may also try just plain salt & water (no idea about salt absorbing into clay in this case).
You may not have to paint it after that, but if you still want, has to be clean first.