It happens on occasion. Hay can mold if not baled perfectly dry and if stored improperly. Luckily this was only one section of a small bale. All the other hay was fine.
If I ever come across hay that is really tightly bound I make sure to inspect it closely. This one was pretty obvious and pretty fuzzy in areas
I figured I'd post the pictures since when I was first a horse owner I was so worried about moldy hay and that I might miss seeing a moldy section and accidentally feed a horse. Well, most of the times it's pretty easy to see. Hopefully this pictures show what to look for.