Not sure its mould, the bark doesn't look soft and spongy, not rotten?
I wondered about wooly aphid, you could try googling it......

Does look more like either woolly aphid or even one of the scale insects. Try washing it off with soapy water. If either of these pests then you need the soap to dissolve the wax covering on them to get at the actual insect underneath.

Picture's not clear enough to see detail, but I think Owdboggys right, its most likely scale, in particular, Horse Chestnut scale, which frequently occurs on Acer trunks and branches too. If soap and water doesn't wash it off, use a disposable cloth, moisten it lightly with methylated spirits and rub that over the area. Check any other woody parts, including fences, even pots, nearby - they often gather on those too.

thanks for that everyone will give soapy water a go cheers