"Coral" hydroids refer to the old group of hydrocorals, which include Millepora (fire coral), Stylaster (rose coral), and Allopora (Pacific). They don't have any stinging junk that gets up into the water; they're all stuck on the bottom.
The "chiggers" getting stuck around your neck could come from all sorts of things, but not coral hydroids. They could be regular hydrozoan jellies, stray tentacles from true jellies, loose dactylozooids (stinging tentacles) from siphonophores like the Man 'O War, chain siphonophore jellies (most siphonophores are this type and they hurt like heck!), oh and cubozoans. Being a Marine it's likely that cubozoan stings are only a minor annoyance to ya'... they kill or incapacitate most everyone else.