I dont have any gold dust days, only giant day and leaftails and banded, but with any hatchling it's always touch and go for awhile, they can appear fine, but may be weak, something wrong we can not see.
I have had other species of hatchlings, appear so cute and healthy, only to find after a couple days that one would die. It seems such a shame that they go through the whole process of coming to BE, and grow and hatch only to DIE
but heck as sad as it is at times, it happens to human babies also which is so devastating.
If all your husbandtry was right on, then I guess he just was weakened by something. Make surr your parents especially Mom, is in tip top shape/plenty of extra calcium,
Hang in there, don't give up 
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PHEve / Eve
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