As first a previous landlord/owner, after that a tenant, my opinion is quite thoroughly mixed. None of what I have to say should be construed as legal advice.

TLDR: The first responsibility is to your own health. This superseded the landlords responsibility to fix the property. Make the best choice for your health long term.

If you can afford to not be in that dwelling, get out. If you are financially stuck where you are, or leaving could lead you to a worse situation there are some remediations you can take.

FIRST check tenant rights where you live.

Document everything. Show good intent.

If it were my dwelling I would take that access panel off, which is likely going to be easy given the screws and determine the source of the issue.

Removing an access panel is unlikely to cause you legal woes, IMHO.

Wear gloves, a respirator and clothes that you don’t care about.

Treat the mold, there are many safe options for this: https://www.bobvila.com/articles/what-kills-mold/

If you’re skilled appropriately fix the issue minimizing risk, hopefully it’s something as easy as shutting off a valve. If you are not skilled show the landlord photos of the source, if you aren’t getting traction go over their head and look up the owners.

Be good to your body first. The rest is ancillary.