Green glowy things

Or, if you prefer, "glowy monsters," for consistency with the naming conventions used in the comic itself (table of contents). "Green glowy things" is preferred, however, because the Geisterdamen have a pretty good menagerie of green, non-glowy monsters of their own. Whatever you call them, they're a particularly disgusting variety of monster found in the Deepdown beneath Sturmhalten, most likely the result of an experiment by Aaronev Wilhelm Sturmvoraus gone bad. They're first seen here, although the next scene shows them to better (i.e., more disgusting) advantage. Unlike their own green monsters, the Geisters want nothing to do with these things, and have walled them off from their underground chamber of horrors. That by itself probably tells you that they're not the kind of thing you want to run into in a dark alley.

Unfortunately, that's exactly what happens when the rescue party (Lars, Zeetha, Dimo, Ognian, Maxim, Sturvin and Kalikoff) opens one door too many while trying to extract themselves from the Deepdown and get back to rescuing  Agatha. Before they can retreat, Dimo gets stung on the arm by one of the GGTs, and his arm begins to turn into green goo -- about the first time we've ever seen a Jägermonster seriously stressed out by physical insult. Oggie deals with this in typically Jägerish fashion: he simply cuts off Dimo's arm, reasoning that as soon as they find a Heterodyne, he (or more accurately "she," as they've figured out who Agatha is by now) can make him a new one. Dimo is perfectly happy with this analysis and carries on just fine with one arm, but it's this exchange that convinces Zeetha that her Zumil is the heir to House Heterodyne. Meanwhile, the severed arm deliquesces into green goo and is lost from the story, which is just as well.

They're probably seen once again later. As Sun Jen-djieh is regaling Gil with tales of his  father's rescue by The Unstoppable Higgs, helpfully providing us with a flashback sequence in the process, something green-glowy-thingish is seen infesting the medical airship. (Dr. Sun thinks they're some kind of biological weapon, which fits.) Somehow Higgs manages to deal with them and carry on. Dr. Sun notes that the ground troops got some "samples" of the critters, so they may have been part of the Battle of Sturmhalten as well, although it's other monsters that are actually seen there.