Yes, It’s Mold
It looks pretty nasty and yes it is mold. The home inspector noticed this mold of course and brought it to the attention of the prospective buyer, and that’s why we are here. A regular home inspection should find mold when it is this obvious, but he did not notice the other mold in this same utility room that was about 10 feet away. If you are concerned about mold, hire a mold expert to do a mold inspection.