It's probably best not to try and come up with a plausible reason why someone would physiologically require mercury and hydrochloride as primary components of the metabolism. Or how many interesting organic mercury compounds such a metabolism could produce (such as dimethylmercury subbing in for urea for an interesting twist).
You could have the atmosphere be a mix of chlorine (probably low concentration to limit reaction speeds), methane and methyl chlorides due to radiation catalyzing free radical reactions that would logically create these. There'd probably be other stuff in there, obviously, but since this is a species that drinks mercury (which would lead me to suspect there being Venusian-style oceans full of the stuff on the planet), plus some source of sulfur, phosphorus and all that other good stuff, but probably best not to get too preoccupied with the chemistry of it all. Anyhow, metabolism based on free radical reactions seems a bit interesting, truth be told, though I'd have to look up the energies of the reactions in question to come up with something reasonably novel, and if there's something I've learned from this game, it's that it's unwise to get yourself this highly invested in what may or may not be a throwaway concept.
Not that I'm not about to do it, obviously.
With chlorine and methane, it'd need to be above 34.04 °C/−29.27 °F (according to Wikipedia). Which is enough to make mercury liquid. So that could work? (Or I could say "screw this" and change the survivability range to have it fit the gas and liquid. Nobody says I can't! and nevermind.
I still need to get done with the missing details of character sheet because of this minor detail, though. For the blood of Armok!)
Also, while I was trying to sleep, I've realized one quite a problem.
Timezones.
You see, when you most of you pop up, it's TWENTY SOMETHING HOURS HERE.
(It's currently 8:45ish here, and Hugo's whooping 9 hours behind).
I had problems becaues of that before.
One RP was vast enough that there was just enough timezones that there wasn't too much problems, but an earlier one was... Well, I could spend few hours or so, but after I left they'd still be here and sometimes there'd be retcons because of fact I can't stay.
I remember joking about how RP are the chaotic evil of RP. (Also, there was that "law" named after someone of the RP that more or less is "that guy you want to RP with won't be available when you are, but as soon as you leave, he pops up" - obviously it was a little exaggerating, but still.
)
So yeah, how do we handle timezones?