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
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