Is there mold in my apartment bathroom? What is causing the unpleasant smell there?
There may be several possible sources of the smell.  First let's address the mold issue.  Generally mold thrives in moist environments.  What is the climate where you are at.  You may need to run a fan to air out that bathroom for a bit.
Second, sewer and toilet area issues can contribute odors as well.  For example, a shower which isn't used may have a trap which has become dry.  Sometimes one will hear water running, having higher frequency splashing noise than water in pipes.  That may happen with a trap, usually a kind of S curve which blocks gasses from entering the house, from the sewer.  That could be a shower or tub, but it could also be a sink.  Run some water down every drain, and see if the odor diminishes.
Third, there can be secondary effects.  For example, the bottom of the toilet usually has a wax ring.  If someone poured hot water down the toilet, it could soften that ring and cause a leak just large enough for sewer gasses. Also, if the toilet were loose in it's setting, that could also permit the development of a crack in the wax ring.
Like toilets, sink drains can also get small leaks, which will permit gas to flow but may not leak visible water.
For a path forward, I would first dry out the bathroom.  I would get rid of any visible mold, whether black or light tan.  Consider a bathroom cleaning product with a mildewcide  in it. The major brands have products like this and you can always call their consumer help line for recommendations.  Run the fan to get the room dry, and to reduce the odor.  Opening windows may help.
Then run some water into every trap (drain).  And then wait and sniff.  See if you can identify where and when the odors appear.
Also, is there an apartment above you?  If the wall which is on the outside has outlets, do you smell the odor near the outlets?
Lastly, there could be a defect in the plumbing in the wall, or even leaking from another apartment adjacent or above you.