Do you know about channeling? It happens when you grind too fine. Instead of an even flow through your coffee bed the flow forms channels of high flow rate. This produces a coffee that is underextracted and overextracted at the same time leading to a sour and bitter cup.

I may be confusing bitterness with acidity

I dont think this is necessarily what's happening here, but lots of people use the term "acidic" when really just trying to say that the coffee isnt sitting well with them, or that it's too "sharp"/strong.

Acidity/sourness/low pH are obviously all almost synonymous, but there are differences in their meaning in beverages. "Acidity" is usually just a characteristic that is neither positive or negative. E.g., an ethiopian coffee might have high acidity, which comes across as fruity. Sourness, is pretty much always a negative, and it's commonly the result of underextraction. Alternatively, it could be that your beans are too fresh, and they just need to be rested a little. Roasting creates CO2 in the beans, brewing releases that CO2 (creating the bloom of a pourover, or the crema of espresso). Dissolved CO2 creates carbonic acid, though, which is sour.

Alternatively, you aren't experiencing sourness but astringency. It has a similar lemon-like kick to your salivary glands, but a different cause. Astringency could be from overextraction, or too high of a temperature.

If you are already at an espresso grind, I would go much more coarse. Also, try to keep the temp down so the coffee comes out slow and steady. Also, if you are using beans within the week they are roasted, maybe wait a week to use them. Lastly, like other noted, moka pots are somewhat hard to use because the physics dont let you mess with as many variables as something like an aeropress. It could be that its just not the best method for you.