JennyC wrote:
Scale insect?

Yes - it must be. There's lots of different one which secrete different
stuff yurk. I've had a persistent one on my maple. All you do with
all of the scale insects, be it unapsis, coccus, pulvinaria etc.
depending on which tree it lives on, a toothbrush and some soapy water
will do the trick. However they all have natural predators like lady
birds, birds and even wasps eat them. I wouldn't bother. Sometimes they
even look quite pretty and interesting )