It seems like all of us have snapped this off at one point or another and usually, it causes panic because it is often the first piece we actually broke, ourselves, while working on our RSX's. It is Idle Air-assist Valve and is the the tee-shaped plastic tree which sits atop the coolant neck on...
Yep, idle air assist. You can get a delete kit for $30 or so or just cut that flush and tape over it if you want a cheap fix. It’s designed to warm the car up faster on cold starts by raising your idle but will still work just fine with that taped up.
I broke mine as well lol, and I found another one on eBay for $50 or try a junkyard. It's called "Air Assist Valve Solenoid Coolant Sensor"
Its from an Accord but its still a Honda so you'll be fine. Do not go to Acura either, they will charge you $200 for a whole kit.