When you re-turn it, all the mold will be cut off. It’s only skin deep.
I don’t think it’s typically hazardous to your health (at least, I hope not!) but if you want it dead, you can spray it with dilute bleach.
DISCLAIMER: Any posts on LJ are posted by individuals acting in their own right and do not necessarily reflect the views of LJ. LJ will not be held liable for the actions of any user.