A non-toxic Zydrate prop! Only glows underneath a black light.
Blue liquid (Kool-Aid)
Tonic Water (You can find tonic water under mix drinks or juices, it's use for alcoholic drinks. but glows a brilliant blue color with the light, but goes clear when away from the light)

Very simple and great to look at :)
The only thing you need is Gravrobber's gun ;)