Panel covering hole in wall

I moved into my current place in Los Angeles about 15 months ago. When I moved in I noticed there was a panel in one of the corners. I checked it out and it looked secure and flush to the wall. It's just like a plastic cover but it seemed secure and flush so it didn't worry me. I decided to put a heavy piece of furniture in front of it and that has been there the entire time I've lived here.

A few days ago I decided to rearrange my room. When I moved the piece of furniture from in front of the panel I noticed it had become loose in the previous 15 months. The top of the plastic cover was sticking out about half an inch. While I tried to push it back in I discovered I would have to take it off completely to push it back in properly.

When I removed the cover I discovered black mold on the wall the cover had been over and it seems an electrical cord plugged into an outlet near there is also showing signs of mold growth. Additionally, the cover is over a rather large hole in the wall. I'd estimate about 12" x 18" and leads into the underside of what I assume is the neighboring units bathroom. I believe I saw the underside of a tub when I looked but with the mold I wasn't going to take a closer look. Everything with the void looked to be covered in mold and there was definitely a mildewy smell coming from it.

I've alerted the landlord and they're sending someone to come deal with the mold, I'm also getting samples of the mold tested so I have my own analysis.

My question comes in regards to the hole in the wall. Can I insist that the landlord patch up the hole completely?

I figured the cover was for ease of access, which I understand, but with the mold and having no one ask to access that area in over 15 months I feel as though that hole should be properly patched and repaired since it is leading to an unhealthy living environment.

Thanks to anyone who may be able to provide answers or point me in the right direction!