Parmigiano Reggiano Mold
where is the mold and what does it look like?
Typically a good wheel of PR does not intentionally have mold on it's exterior. Once cut however all cheese will develop mold on its cut surfaces eventually - this is normal and natural.
But because this cheese is so firm and dense you can safely cut away the mold and eat the remainder of the cheese. Make sure you cut all of the mold and a bit of the surrounding area of cheese to be safe.
But be warned once mold starts to grow, it tends to come back quickly even after trimming. That's why it's important to handle a large wheel/ piece with very clean hands or better yet try to not touch the surfaces with your bare hands at all by using a piece of clean wax paper or plastic over your hand to touch the cheese and handling byt the rind end is always a good idea if long term storage is needed.
The hard, dry rind of PR does not typically support mold growth, but it is not unheard of. I have seen black mold on the rind of a cut piece of PR wrapped in plastic and left that way for a long time. I suspect that there was moisture trapped between the plastic and the rind that led to growth of the mold.
Yes, what does it look like?
A good parmigiano reggiano will have little white spots that are crystallized amino acids. After it's been cut the surface of the cheese darkens a little, making the white spots more obvious. (I did a search and there are actually several previous discussions on chowhound on just this subject!)