When I think of "tracer" and marbles, I think of a Peltier Tracer. But can't call the police on you for calling that a tracer ... it's a descriptive word.
I'm not sure if collectors have a particular word for it. Maybe someone else will know.
P.s., I would just call it a Transparent Swirl. It's really hard to figure out which WV swirl company (or possibly Alox in Missouri) made them.