a slain vampire in Forlorn Gist /way -151, 18, 359 is showing "Grassblade was here" pretty much since expansion came out. I think it may be a bug, hard to believe this person has marked it every time it's open since expansion launched.
That is flavor text of the zone itself. It's a joke that you fell for. "Grassblade" leaves his mark on other Killroy Was Here zone flavors elsewhere in Vetruvia. The achievement for marking all the markables in FG does not include any vampire corpses. In fact, one of the actual markable items is right there next to the slain vampire.