Hi. We live in an apartment complex built about 20 years ago.
We've noticed that black gunk builds up relatively quickly (I would say it is noticeable within a week or less) in one of our toilets (we have two, doesn't seem to be bad anywhere other than the one toilet).
No mold in the tank and only a small amount in corners of shower after a few weeks (seems normal)

It looks like black liquid dripping down the sides of the toilet (mainly on the left side). I forgot to get a photo before cleaning the last time.

This is the bathroom we both shower in daily and it does not have a vent fan to the outside or a window (just one of those cheap fans that blows air around)

Does this mean we have mold in our pipes, or could it just be humidity in the bathroom? The relative humidity in the apartment is usually 45-50%

I am worried there is mold in our drinking water. I have been having quite a few unexplained health issues.

Is there a way to test the water for mold or any other way to get to the bottom of this? Is there a way I could flush or clean the system on my own?

The maintenance and management here is absolutely horrible. Ideally I would need some way to prove there is mold otherwise they won't do anything.