[Falina]

could be a wee fungal infection if the white stuff looks kinda cotton-wooly?

 

[Falina]

if its only a wee bit then thats probably not what killed it. i had 2 mollies with a fungal infection and they had a fair bit of it on them. i treated them with "aquarium treatment 8 - anti fungus and finrot" and they recovered 100% within a few days

sorry about your fish RIP