It may just be Lichen which is pretty harmless so long as it doesn't completely cover every stem and branch as this could block the breathing lenticels on the stems. Normally not a problem in small ammounts though a picture would help.

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Thanks for your answer. The mould does seem to be just on the main bark but I have noticed that it has spread a bit, but not on to the stems at the moment. To be honest the tree is looking really great this year (the best it has ever looked) and I just don't want it to go down hill.

Hi Vectra06, is this agent in question fluffy or flaky and what colour is it?

Hi Fractal, it is like a sticky white substance that I wiped off with a damp cloth over weekend but it seems to have come back, so I dont know if it is something a bit more sinister. I noticed some other people have simlar problems with their trees also and it looks like I may have to use soap and water followed by meths on the bark. Does this seem the right thing to do?

If it is coming up from the soil, it may be an innocuous and harmless slime mould. These live on organic material in and over the soil and would be no harm to the Acer. The only slight concern (even though your Acer seems to be in fine health) is that the surface of the compost might be staying too wet?

Wish I could inspect the plant myself.

Thanks again Fractal, but the mould is on the bark of the Acer and does not seem to be coming from the soil, however I was wondering if I could be over watering the tree. Just as an after thought could the mould be a insect lava as when I wipe it of the bark seems clean underneath.