That is actually Euonymous, and you have scale, extremely common.
Chemical pesticides are most effective, but some repeated applications of soap or horticultural oil should knock it out if you want a more natural route. I'm unaware of insect controls that would be effective.
How serious is it (the bush seems pretty fully infected) it's a pretty bush/tree, but was here when we moved in and isn't essential. Is scale likely to kill it and will it spread to other trees. (The holly which is intertwined with the Eunonymous doesn't seem to be affected).
Any advice, could I just leave and heavily prune in Winter?