I've had to deal with moldy hay occasionally, usually a flake or two. The 10 bales of hay that I got awhile ago is turning out to be not as consistent and nice as I would like, hence the reason I didn't buy more than 10 and decided to shop elsewhere for my year supply.
The most recent bale that had mold was kind of hard to see at first, I open each flake to examine the hay when I feed so since this bale was more crumpled feeling than just neatly packed that gave me the first hint there could be an issue lurking. I noticed the mold in parts that I opened out, so immediately went into dissection mode. I pilfered through the next couple of flakes, anything suspicious looking was thrown in a "toss out" pile. The bale got a little bit better towards the other end but I pulled apart every single flake anyways. Pretty much most of the bale got disposed of.
I placed the bad flakes into a wheel barrel and started sweeping up the area to make sure I got all the bad hay bits. Of course Dani was used to eating hay out of a wheel barrel from Whispering Winds. They would load in the hay with wheel barrows to haul out to the field and make piles for the horses but Dani would never wait, she always went straight to the wheel barrow to begin her meal. So with the moldy hay in the wheel barrow that was an issue! I kept pushing her away but she kept coming back. She was so pathetic looking and eyeballed me as if to say "mom that's my hay, what's your problem?". I finally ended up finding a couple good flakes put it over by her bin and tied her up to the fence loosely so she could eat the hay and I could get the bad hay out of the stall without a chestnut muzzle getting in my way.
I guess all in all it's not too terrible to occasionally have to throw out some hay. I've had several bales of hay from these 10 bales that I purchased. The 30 bales of Timothy I bought in May were really good and consistent. There was maybe one bale I was a little worried about and had about one flake that was sort of moldy, so I probably threw away about 4 flakes of that bale. I think it's just safer to throw out more hay than to accidentally feed hay that is moldy.
I'm looking forward to starting her on the hundred bales that I recently purchased since they are really good bales of hay and look like they're gonna be nice and consistent. I think it's better to be meticulous about your hay and throw any suspicious hay out, than to have to call out the vet on a colic issue!