I feed haylage for all of ours due to my mare needing to have her hay soaked and find haylage easier to manage, and have only ever had a couple of bad bales and as soon as i opened them i knew they werent right. they were hot, slimy and smelly!!
We sometimes get white in the middle of the bale, but as with hay, if it smells ok its fine to feed. My supplier, who is abit of a character, says to feed it anyway as the horses will pick out the best but at £30 a bale i demanded, and got, a replacement. Never knew the white was sugar though, learn something new every day!
The bottom line is, as with hay, if in doubt dont feed it. I would rather pay for an extra bale of haylage than a vet bill for colic, coughs or anything else caused by feeding dodgy forage.