That's the water neck coolant outlet, I literally did the exact same fix a couple of weeks back (mine was pissing out red though :P). Basically you just need to get the 2 bolts it sits on (be really careful mine didn't want to come and I did a few doses of WD40) and I think I undid a third to make it easier to get at, pulled off a few lines etc. About half a litre of fluid came out and then I just topped it back up and bled it, didn't get any bubbles and seems fine.
90301-40005 is the product number, the stock one has 6 bits around the inner circumference but I just used a thicker oring and put some of the goop on the oring (Couldn't be bothered waiting for the oem part to come in the mail), I did end up filing it a bit as it wouldn't go in quite properly at first. The oring that I pulled out was literally in pieces. Now the cowling underneath of my engine is not covered in red fluid which can only be a good thing.