What is this leak outside my apartment?

Edit: Photo in comments

Hi all, for the past two weeks or so this panel outside my apartment has been leaking (?). At first it was a very light stream that would dry up within hours and I didn't think much of it. I thought it was some issue with the building or my neighbour but nothing related to me (I'm still not sure if it's the case). I have checked my apartment many times and there's no indoor leakage.

Last week on thursday there was a letter taped outside my door from a water leak company saying that the apartment below mine was complaining of a leak. I called them and they came to check my apartment that same day.

The poor man spent over 2 hours inspecting my apartment. He removed my washing machine and looked at the kitchen sink for ages but didn't see any issues with it. Then he moved on to the bathroom and just said that the water pressure in my toilet was a bit weak and he fixed it. I asked him about the issue with the panel outside and he said he's still not sure if the issue is from me or from the apartment upstairs and that he would check it later (not sure if he did).

Basically he left saying that everything in my apartment is now fixed. The issue is that the leaking outdoors has only been getting worse, it's still happening, and now there's dirt or whatever that dark stuff is mixed with it.

I've tried to talk to the building 管理人 but he's an old ojii-san and it's really hard to communicate with him with my limited japanese. He doesn't seem to have a clue what's going on either.

I'm a bit at a loss. Should I call the water company again? Is the problem even coming from my apartment? If anyone has a clue, it would be greatly appreciated.