I'm skeptical about highlighter liquid glowing. Do a quick test before you count on that idea.
Wildfire Water Dye is the product that a big time production would use. It's $28 for a pint which tints 275 gallons of water. Check out this picture from their site of glowing "water
curtains"
Secondly there is the problem of getting enough UV light on
stage for the stuff to glow. If you can black out the whole set and just fire up the black lights then you won't have a problem but as soon as you start using other lights it gets REALLY tricky. You can't just use the cheap black light lamps at the Halloween store. You need some serious firepower to really make stuff glow. Which probably means renting. This trick is going to get expensive quick.
I do like the idea of a thickener... but being horrified by Dereks' product, I would go for a food grade thickening agent. There are lots of older people out there who
choke on liquids. They use a thickening agent that turns a glass of water into
clear pudding. You should be able to find several options at your local drug store.
A cheaper and probably just as effective approach would be colored lights sealed under glass with a thick oozy
clear liquid on top. The
clear thick oozy liquid would
pick up the color and look great (be careful about having good ventilation for the heat of the light).
Cheapest/easiest option would be thickening agent, some green food coloring, and forget trying to make it glow from below.