Dead colony,

[stuart]

Dead colony, the end frame covered in a greyish brown dust. Any ideas?

 

[Hivemaker.]

Moldy outside comb.

The bees died of starvation.

 

[MuswellMetro]

or wax moth, they can cause a saw dust type detritous

 

[Eyeman]

Brown dust is common in stored brood combs without wax moth. I've assumed it is due to the pollen mites.

 

[Erichalfbee]

Brown dust is common in stored brood combs without wax moth. I've assumed it is due to the pollen mites.

At least they get the pollen out of your supers. I didn't get all mine on last year so I left the ones with pollen. Checking them this spring at the bottom of the stack is brown dust and all the pollen had gone or was readily shaken out
Back to OP how many bees where were they on the comb were there stores? Agree about the mould

 

[TryingToLetThemBee]

I've had terrible trouble in a couple of overwintered double brood colonies with lots of mouldy comb AND weak colonies. I think the winter conditions were to blame and I probably had inspection boards in too often. I realise that mould and bees are not a good mix, and maybe that's what happened here, perhaps with a shook swarm being the cure if it is caught earlier.