Very few of the common parasites can be conclusively identified from pictures of dead fish, simply because most of those parasites leave the fish as soon as it is dead. The only one who stays to devour the dead body is Uronema.
The timeline of 2 - 3 days doesn't fit ich or velvet. It could fit Brooklynella as that one hasn't to go through any cycle to reproduce, but just multiplies by simple division on the host.
The white slime would fit this one also.
Another option could be a bacterial infection.
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