white foamy spots appearing on my Acer's Trunk?

Cuckoo spit - bubbly stuff produced by leaf hoppers? Harmless and can be washed off.

sounds like scale insect. did it have a small conical brown cap? can be removed but it takes an age.

If it's foamy, I'd go for cuckoo spit too. In itself, it's harmless, but inside will be a small bug called a frog hopper. This sucks sap and can cause new growth to curl and warp, but will not kill the plant. Easily removed by hand or hosing off.

However, scale insect as SBG says is also a possibility, but they're not 'foamy'. They're more solid and white looking.

Have just been in the garden. Identical white gooey stuff as you describe on the stems of Rosa rugosa and I am pretty sure now that it is the egg-containing deposits of scale insects. Very like melted marshmallow! Yuk. Brown scale insects found on lower leaf surfaces too.

cookoo spit

melted marshmallow is a good description of the scale offsprings protective goo.