Its anything that makes a bubble, SUCH AS the oil bubble, the golem nr 3 reflect / heal skill and any other skills that make a dome that either heals or reflects projectiles.
Ive been having bugs with dome graphics like the bubble from oil in WvW since todays update.
I thought my GPU was dying at first.
https://ibb.co/h2TzysL
It does seem to be an issue with Win 10 Fall Creators Update concerning problems with DirectX. Try the following as a fix.Right click your GW2 exe, choose Compatibility and check Override high dpi and disable Full screen optimizations.Hope this works