One of the basic foundations that sets polyamorous folks apart from swingers and whatnot is the slightly rabid belief in communication. Asking poly folks if you should talk or shut up is kind of like asking a hungry shark if it would like to bite a wounded bloody fish or nurse it back to health.
Nom nom nom!
Seriously though, you have the best vantage point on how much opening up to this guy may go off the rails. If it's all sorts of ruin with a tiny sliver of hope, maybe quiet is better.
However, if you've talked to your target (H) about friendships and drama in regards to your partner (C), then H already has some understanding of your situation. Your memory of how he handled that discussion may help inform your thinking about whether to proceed. It sounds like you are already halfway down that road.
You might consider something simple like, "I wonder if you'd be flattered if you knew I had talked with C about you?" Or "I was thinking about how you understood that it was important for poly folk to not mess up friendships even when they're interested in a guy..." Ok, maybe you won't say "folk." I say folk a lot. You might try it though--to yourself out loud... right now. "Folk." See? Fun.
Alrighty then. Onward, Castalia!