It looks like some sort of insect or a caterpillar larvae something like that.

Oyster scale insect. Bad infestation there too. Difficult to get rid of. The white is a water proof coating so insecticides do not reach the insect. You need to spray with a Horticultural soft soap type insecticide. Or if it is only that one area brush them off with methylated spirits. Nasty things though and you will need to repeat the treatment until they are all gone.

The white things look like Cottony Maple Scale which, according to this website:

https://www.treecarescience.com/tree-problems/insects-mites/cottony-maple-scale-diagnostic-guide/

can also be found on hawthorn.

Cure is the same whichever scale insect it is.