Before you start please make sure you have the following items on hand:
Wear your 3M N-95 mask. Mold spores can cause serious allergic reactions, coughing and inflammation in your lungs.
After you are done cleaning up you should spend some time thinking about why it happened. This is an important step in preventing it from happening again.
This is a problem I wish Ikea would address by doing a better job of sealing both sides of the backboard and to stop using paper based tape and glue material.