I live on a dairy farm, so I've always been taught that white mold is alright, but green/blue etc is definitely not.
White mold is just yeast forming, and horses will eat it just fine, however I prefer not to feed it anyway, if it is in a piece of haylage, I throw it out and forget the rest of the bale (I pick bits off the bales dad feeds to the cows anyway), because the bale has to be monitored carefully so as not to make my horse get the runs anyway.
In a bag of feed, if it is SMOOTH white "mold"(yeast), I will still feed it, if it is only a little bit. As soon as the feed starts to look gray from mold in the center, nah, its gone. I have noticed if I don't throw it out (eg if I leave it in the shed unfed) it goes all slimy and disgusting rather quickly.