Mold due to slow leak

We have a slow leak near our furnace which just so happens to be in a closet by my daughter’s play room. I think it’s been happening for a little while because I’m pretty sure I see some mold. Has anyone had anything similar happen? How concerning is the mold from a health standpoint? I will obviously get our landlord to clean it up ASAP but I’m a bit concerned in the meantime.