Hi Krk and welcome to GoY. Having turned the laptop sideways to look at the image I have to say that doesn't look like mould to me especially as it is also on the supporting cane and the pot itself. I think it is horse chestnut scale which does affect acers, http://apps.rhs.org.uk/advicesearch/profile.aspx?PID=650

Definitely scale infestation - slightly dampen a cloth with methylated spirits and rub over the woody parts, removing the scale as you go. Do the same to the bamboo cane (where I can see some more) and the pot if there are any present there, but don't apply the cloth to the leaves.

Fantastic, thank you Moon Growe and Bamboo for your help. Off to get methylated spirits! Kate