Egg white - mouldy?

[LeonardoAcropolis]

Hello, quick query - I've had a single egg white stored in an airtight box in the fridge since saturday. Would it still be ok to use for meringues?

I would say so. Provided it looks and smells ok. There may be a risk of some bacteria being present but they will get cooked off in the baking process and rendered harmless, so I would be comfortable with it personally. :)