Mold forms on sweet miso -- can I still use the miso?
The mold is the indication that the content is compromised with substances that the mold can grow on or with mold that does not mind the environment. This will happen for example when you use a spoon that is not clean. Unfortunately, there is no way to know if this substance is only on the top of the miso. So, whatever it was that the mold grew on, could be in lower layers, too, where non-visible mold with non-detectable toxins could have developed.

So, while a lot of food stuff lasts nearly forever, this is only true while it is uncompromised and proper hygiene is followed.

So, my recommendation would be to toss the whole jar and learn from the experience and not treat fermented food as invincible and reconsider package sizes.