If the mold is just on the surface, it is caused by condensation

In that case, check if there is an exhaust.
If there is an exhaust fan, he should use it.
If the fan is not working, he should report to you, not wait until mold grow up.
If the mold grows inside drywall, probably there is leaking from the roof or upstairs.
In that case, the tenant is not responsible, and you need fix.
Either way, I would not let tenant repair it.