use mold resistant paint,

clean it up with bleach, there is no need to replace sheetrock unless you have water leak inside. use mold killing primer and mold resistant paint like this

http://www.homedepot.com/p/Zinsser-1-gal-Mold-Killing-Primer-2-Pack-276049/205009921
http://www.homedepot.com/p/Zinsser-1-gal-Perma-White-Mold-and-Mildew-Proof-Eggshell-Interior-Paint-2-Pack-2771/202760948