Do you think you could edit your question to add a photo please Linda? It may be nothing to worry about - it could just be lichen, but without a photo we can't really tell.

Have you repotted recently? It is possible that the trunk is too deeply covered with soil and getting moist - allowing mold (or Lichen as Beattie has said) to grow.

Sounds like scale insect - there's a lot of it about this year, particularly on Acers. If you've got any methylated spirits, moisten some cotton wool or a soft cloth with it and rub the fluffy deposits and the scales off, making sure you get them all wherever they are. Otherwise, a mild soap and water solution, dip a cloth, wring out and clean off with that, preferably a cloth you can bin afterwards.