Paul, I think Bamboo’s original summation is correct. It looks like a scale insect. These are insects within a group of “sap sucking insects”. In the case of scale I sect, they attach themselves to the stem with a mosquito like mouth poked into the branch or leaf. They aren’t able to digest all of the sugars that they consume. As a result, they excrete the remaining undigested sugars as frass, a technical term for poo. The white material in your photo will be this frass. You could try tasting it to see if it’s sweet. It shouldn’t be harmful but I leave that decision to your discretion.
If you ever see a trail of ants marching up your trees, the chances are that there is a sap sucking insect attacking the tree. There is a strong relationship between ants and aphids. This sounds charming but aphids will inject their probosces (mosquito like mouth) into a rose stem or bud and suck the sap. At the same time ants will feed on the undigested sugars from the aphids’ rectum. It’s a win win relationship because the ants get a free meal and the aphids aren’t compelled to change feeding location.
If you ever see sticky shiny leaves on a plant, this too is a sign of sap suckers operating without ants, where the liquid frass drops onto the leaves. This can also result in sooty mould growing on the leaves. This mould doesn’t attack the plant but feeds on the sugars left by sap suckers. The net result is unattractive but also stops sunlight from reaching the leaves and photosynthesising where the sooty mould is.
By all means use alcohol to clean them up but there are other treatments for larger infestations. It is worth knowing a little about the anatomy of insects if you are trying to kill them. Insects are too small to have lungs, so they have microscopic breathing holes down the sides of their bodies. Oxygen diffuses in and CO2 out, simply as the breeze blows past. If we can block these holes with a relatively form of low toxicity preparation, we don’t have to restrict our children or pets from the garden.
Commercially available White Oil, Winter Oil, Summer Oil or Pest Oil are all the same material available from your garden centre. These are water soluble oils that you can dilute (according to the label) in a small hand sprayer, the sort of sprayer that you might use with water when ironing. If you spray the insect with the product with a good coverage, you will block the breathing holes and the insects will die. Alternatively, you could fill your spray bottle with water and add a few drops of dish washing detergent and achieve a similare result.
Scale insect like that on your Acer will not drop off just because they are dead. There mouth is still stuck under the bark. So small infestations are probably better dealt with using a rag, as Bamboo suggested. But if you have scale insect on your Acer, check your garden for other possible sites.
While you are around you Japanese Maple, check for snails in the branches. They will eat the bark and ring bark an Acer palmatum. Or eat enough of the bark to seriously compromise the health of the plant. In my experience this is more of an issue in spring but your damp summers might extend the period of attack.